The Hunger at Loch Avon

I sat at my desk in the small timber cottage near Grantown-on-Spey, with Callum Reed's letter spread open before me. The paper was creased along the folds after three weeks of opening and closing it, as if reading it once more might change what was written there. Outside, snow lay thick over the Scots pines, and the white ridges of the Cairngorms disappeared into a lowering sky.

It was late January 2019. The wood-burning stove kept the single room warm enough, but the cold still found its way through the window frames. Twenty-two years of guiding people through the Scottish Highlands had prepared me for storms, injuries and panic. I had seen red deer charge when cornered, helped carry walkers with broken legs off remote hillsides and once talked down a man suffering a complete psychotic collapse after eating mushrooms he had found beside a path.

Nothing had prepared me for what happened beside Loch Avon.

Nothing had prepared me for Rebecca Voss and those students from the University of Sussex.

I got up and made coffee. The ritual helped to settle my thoughts. The cottage was not much: one main room, a sleeping loft, a stove and a view across the snow-covered mountains that compensated for the lack of comforts. It gave me somewhere quiet between guiding seasons, and usually that was enough.

Back at the desk, I opened the filing cabinet and took out a folder labelled University of Sussex - Cairngorms Field Trip - September 2018. Inside were all the documents from that week: contracts, medical forms, emergency contacts, route plans and copies of official statements.

"Might as well start at the beginning," I muttered.

At first, it had seemed like any other university booking. Sussex needed a guide for twelve undergraduate students and one academic trip leader. They wanted a week-long Cairngorms ecology field course that would contribute to a university module, challenging enough to provide meaningful field experience but requiring no climbing or other technical mountaineering skills.

It was standard work. I had led dozens of university groups, and the bookings paid well during the gap between the summer rush and the winter season. When the first email arrived, I checked my diary and found I was free.

The timing looked ideal: early September 2018, after the busiest tourist weeks but before the first serious winter weather. Conditions would be cool, the paths quieter and the daylight still sufficient for the route we had planned.

I remembered reviewing the medical forms with professional detachment. No severe allergies. No respiratory condition likely to cause serious trouble on the hills. Two students had minor dietary restrictions, but nothing that complicated the meal plan. Most appeared reasonably fit and listed some experience of walking, camping or organised sport.

The university liaison, Dr Alistair Fenwick, had stressed during our telephone call that they were not a party crowd.

"They're serious biology students," he told me. "This is part of their environmental science programme. They're going to learn, not to get drunk halfway up a mountain."

That was always welcome news. Few things were worse than supervising undergraduates who treated a remote landscape as an escape from university security and an excuse to get hammered.

I took another sip of coffee and found the student list. Twelve names. I tried to match them to the faces in my memory. Some returned at once.

Callum Reed, tall and sandy-haired, with a camera almost always hanging from his neck. Owen Price, athletic and outwardly confident. Hannah, Bethan, Ravi and Lucy. The remaining faces were less distinct, blurred by the hundreds of clients I had guided over the years.

And then there was Rebecca Voss.

Her name had not appeared on the original paperwork. Dr Fenwick had been due to lead the trip himself. Two weeks before the group arrived, I received an email explaining that he had been admitted to hospital with a severe chest infection. A doctoral teaching assistant named Rebecca Voss would replace him.

I had no reason to question it. Lecturers withdrew from field trips all the time because of illness, family emergencies or departmental demands. A younger academic leader could sometimes make things easier. They often related more naturally to the students and had more energy for long days on the hill.

I unfolded my map of the Cairngorms and traced our route towards Loch Avon. We had planned to establish a base camp near the loch and spend three days examining the surrounding corries, upland habitats and geological features. The place was beautiful and severe: cold water beneath steep mountain walls, with open ground exposed to weather that could change in minutes. I had guided groups through the area many times without a serious incident.

Callum's letter lay beside the map. Even when I was not looking at it, I could see the words.

"I don't know what's happening to me.

"I get these hungers. Regular food doesn't help. Last night I attacked Lucy. I don't remember much, only this overwhelming need.

"I woke up covered in blood. She's in hospital now. I told everyone it was a large stray dog. I said it came through the unsecured rear door. They believed me, but I know what I did."

The letter was dated 28 December 2018. It had arrived in early January, about three weeks before I sat there trying to decide whether to answer it.

I closed the folder. Sometimes I wanted to burn everything: the letter, the documents and every physical trace of the memories. But destroying evidence would have crossed a line I was not ready to cross.

After the incident, I had been careful about what I told Police Scotland. I had not wanted to hide anything, not at first. I simply knew how the truth would sound.

The academic trip leader had changed into something animal-like and attacked her students. One of the students had died, risen again and tried to kill the rest of us.

They would have sent me for psychiatric assessment before I finished the first sentence.

So I edited the truth. I told them Rebecca had suffered some kind of violent psychological breakdown, attacked several members of the group and fled into the mountains. It was close enough to what had happened that I could repeat it convincingly. I left out only those parts that would have made me sound insane.

Later, when the UK national public-health and security authorities became involved, the edited version was no longer merely convenient. It became the account I was expected to endorse.

I walked to the window. Snow was falling harder, broad flakes drifting between the dark trunks.

In a few weeks, I meant to close the cottage and leave for seasonal winter walking work in Dartmoor and the Brecon Beacons. A change of scenery usually cleared my head. I doubted it would help this time.

The folder remained on the desk, a physical record of something I could not explain, something that challenged everything I believed about the natural world and what might exist within it.

I had to begin at the beginning. I had to try to make sense of it.

That was what I kept returning to: it had been a normal booking. The university had good references. The students had credible outdoor experience. On paper, everything was exactly as it should have been.

A group photograph had arrived with the original paperwork. In it, the twelve students stood smiling outside a science building on the University of Sussex campus in Brighton. They looked clean-cut, eager and impossibly young. Bright futures waited for them. They had no idea what was waiting in the mountains.

Neither did I.

Before that trip, I would have said I knew the Highlands. Twenty-two years of leading people through remote country had given me confidence that, in retrospect, bordered on complacency. I knew the paths, the weather and the behaviour of the wildlife. I knew how to manage a frightened group and how to bring injured walkers home.

Nothing in my experience had prepared me for Loch Avon.

I returned to the desk and opened the folder again. If I was going to understand Callum's letter, or have any hope of helping him, I needed to remember everything as it had happened.

Not the sanitised account I gave Police Scotland. Not the distorted fragments from the nightmares that still woke me sweating.

The truth, as nearly as I could recall it.

It began with a routine airport collection.

On an early September morning in 2018, I drove a hired seventeen-seat minibus to Inverness Airport to meet the University of Sussex group. I arrived early, as I always did for clients. Professional punctuality had served me well throughout my career.

I parked in the short-stay car park and went into arrivals carrying a sign with UNIVERSITY OF SUSSEX printed in neat block capitals. Their domestic flight from London Gatwick landed on time.

I watched passengers emerge, studying their faces against the group profile I had reviewed. University students were generally easy to identify. They travelled in clusters, laughed a little too loudly and often wore sweatshirts bearing their university's name.

This group was no exception.

The twelve students appeared together, weary from the flight but alive with the excitement of being in the Highlands and far from their familiar south-coast campus. Some wore walking boots softened by years of use. Others had new outdoor clothing whose folds still looked as if they had come straight from the shop. It was the usual field-trip mixture of experienced walkers and enthusiastic novices.

The woman leading them caught my attention immediately.

Rebecca Voss was younger than the academics who usually accompanied these trips, probably in her late twenties. Dr Fenwick had described himself as an ageing biology lecturer with unreliable knees; Rebecca moved with an athletic spring in her step. She saw my sign and came directly towards me.

"Martin Hale?" she asked, offering her hand.

Her grip was firm and assured.

"I'm Rebecca Voss, the doctoral teaching assistant filling in for Dr Fenwick."

Up close, I noticed how alert her eyes were. They moved continually, examining me and everything around us with intense interest. Her dark hair was tied in a practical ponytail, though several strands had worked loose around her face. She wore walking trousers and a lightweight technical jacket. At least in terms of equipment, she knew what she was doing.

"Good to meet you," I said. "I hope the flight was all right."

"As all right as flying ever is," she said with a laugh. "Everyone's excited to be here. Let me introduce you."

She beckoned the students closer and ran through their names faster than I could retain them. I nodded to each in turn, knowing it would take a day or two to connect every name reliably to a face.

Callum Reed stood out immediately. He was tall, with sandy hair, and looked comfortable in his outdoor clothing. Beside some of the others in their stiff new boots and spotless jackets, he appeared as though he genuinely spent time outside.

"Callum's our keen bird and landscape photographer," Rebecca said, briefly placing a hand on his shoulder. "He's promised to document everything."

Callum smiled and raised the camera hanging from his neck.

"I've wanted to photograph the Cairngorms for years."

After the introductions, we went to baggage reclaim. The students gathered around the carousel, calling out one another's rucksacks. Rebecca moved easily among them, spotting luggage, joining in their jokes and keeping them organised without seeming to make an effort.

I noticed how the students responded to her, particularly some of the young men. They stood straighter when she approached and laughed more readily at her remarks. That was not unusual around a young and attractive teacher, but assessing group dynamics had become instinctive for me, and I filed it away.

Once all the luggage had arrived, I led them out to the minibus. The students loaded their equipment into the rear while I supervised, making certain the weight was distributed properly and nothing could shift on the road.

"Find yourselves a seat," I called. "It should take about fifty minutes to reach Aviemore, depending on the traffic."

They settled in, plugging in earphones, checking their mobiles and leaning against the windows. Rebecca took the front passenger seat.

"Do you mind if I sit here?" she asked. "I'd like to pick your brains about the area."

"Not at all," I said as I started the engine. "That's part of what I'm here for."

We left the airport and passed through the outskirts of Inverness before joining the A9 south towards Aviemore. In the mirror, I saw that several students were already dozing. Callum sat near the back with his camera out, photographing the Moray Firth lowlands and the wooded slopes leading towards the Cairngorms.

"How long have you been guiding here?" Rebecca asked as we joined the A9.

"Twenty-two years. I started after university and never found a reason to stop."

"That's remarkable. What's the most dangerous situation you've faced?"

Clients often asked that question, but she sounded genuinely interested rather than merely polite. I told her about a sudden blizzard that had trapped one of my groups on an exposed ridge for two days. We had dug emergency shelters and rationed our supplies until visibility improved enough for us to descend.

"How did you keep everyone calm?" she asked, leaning slightly towards me.

"People take their lead from the guide. If I panic, they panic. If I remain steady, they usually do as well."

"A useful principle," she said. "I'll remember that with this lot."

For most of the journey, she continued asking questions. She wanted to know about the local geology, habitats, weather systems and the conditions we might encounter at higher elevations. I appreciated her interest. Too many academic leaders treated field courses as an opportunity to switch off while the guide took responsibility for everything.

We stopped briefly at a roadside service area. Rebecca moved from one cluster of students to another, checking that they had water and asking how they felt after the flight. She spent longer with Callum and Owen Price, an athletic-looking student whose name I had managed to remember. There was nothing openly improper in the conversations, but the energy she directed towards the young men was different from the manner she used with the women.

Back on the A9, the landscape rose around us. The lowlands gave way to wooded slopes, heather and the first imposing views of the Cairngorm mountains. Students woke, lifted phones to the glass and pointed out ridges to one another.

"I've always wanted to see the Cairngorms," Rebecca said, looking through the windscreen. "They're more beautiful than I imagined."

"Wait until you're out on the paths," I said. "Photographs never quite capture the scale."

"What should we expect at the hostel tonight?" She took a notebook from her bag. "I want to make sure everyone is ready for tomorrow."

I outlined the plan: check in at the youth hostel in Aviemore, hold a short orientation meeting, eat in town and get an early night before the first day of training.

"Perfect," she said, making notes. "They need proper food and sleep after travelling."

The mountains seemed to gather around us as we approached Aviemore. The students became more animated, taking pictures and trying to identify features through the windows. I heard Callum explaining the formation of the corries to several students sitting near him.

We arrived shortly before dinner. Aviemore was still busy despite the end of the peak summer season. I drove through the town to the youth hostel where I had booked the group.

"Everyone take your own bags," Rebecca called as I parked. "Let's make a good impression on the first day."

The students climbed out and retrieved their equipment with only minor confusion. Inside, I dealt with reception while Rebecca organised the dormitory arrangements with a clipboard and marker.

"Orientation in the common room in thirty minutes," she announced when everyone had been assigned a bed. "Have a shower if you need one, but don't be late."

She took charge with practised ease, and I felt relieved. An overwhelmed or disengaged academic leader could double my workload. Rebecca appeared competent, organised and sincerely invested in the course.

As the students dispersed, Callum approached her with a question about his camera equipment. I was too far away to hear the details, but he seemed less awkward with her than some of the other male students, who became tongue-tied whenever she spoke to them.

I unloaded my own equipment and checked every row of the minibus to ensure nothing had been left behind. By the time I finished, most of the group had settled into their rooms. Rebecca joined me near reception, her clipboard tucked beneath one arm.

"What do you think of them so far?" she asked.

"They seem sound. There's a reasonable mixture of experience, and nobody looks likely to struggle badly with the terrain we've planned."

"They're good students," she said. "Serious about their work. This isn't simply a holiday for them. The field course contributes to their environmental science module."

"That's what Dr Fenwick said. Academic motivation always helps."

"Exactly." She smiled. "Though the scenery doesn't hurt."

By the time we assembled for the orientation meeting, I felt confident about the week ahead. The students appeared to have recovered from the journey. Rebecca was proving capable and engaged, and the weather forecast for our route into Cairngorms National Park was close to perfect.

I could not have known that, within a week, I would be fighting to bring those same twelve students out alive.

Nor could I have imagined that the enthusiastic young teaching assistant sitting opposite me would become something I still cannot fully explain.

The last daylight faded beyond the hostel windows as the orientation ended. I gave each student a map of our planned route and answered questions about the paths, terrain and wildlife.

"Get some rest," I told them. "Equipment checks begin at 08:00 tomorrow, followed by a practice walk. I want to know that your boots are worn in and your rucksacks are adjusted properly before we commit to the approach."

They collected their maps and notebooks, speaking quietly with the fatigue of the journey beneath their excitement. As they left the common room, Rebecca walked beside Callum, her head inclined towards his while she asked about his camera equipment.

Morning brought the crisp air and slanting golden light that make the Cairngorms so striking in early September.

After breakfast in the hostel dining room, we assembled in the car park for equipment checks. I moved from student to student, inspecting boots, tightening straps and making sure everyone understood how to use what they carried.

"Your hip belt needs to be tighter," I told Hannah, adjusting the straps on her rucksack. "The load should rest on your hips, not drag on your shoulders."

Across the car park, Rebecca was helping several male students. She stood close to Owen as they reorganised his rucksack, leaving her hand on his arm while she explained how to balance the load. When she went to adjust Callum's shoulder straps, she stepped directly behind him and reached around his body to pull the buckles near his chest.

"Let me get that for you," she said.

She remained pressed against his back longer than the task required. Callum shifted his weight and took a small step forwards, creating space between them.

It struck me as slightly unprofessional, but not yet something I felt justified in confronting. Some instructors were naturally more physical in the way they demonstrated things. If the students appeared comfortable, I told myself, it was not my place to interfere.

Our practice walk followed a moderate path above Aviemore, climbing high enough to give broad views across the valley. I maintained a deliberately slow pace, watching how each student managed the ascent and stopping to correct poor rucksack adjustments or help anyone whose breathing became strained.

"Pace yourselves," I called. "This isn't a race. Move steadily and you'll arrive with energy left."

Rebecca stayed near the front at first, then repeatedly changed position to walk with different male students. She laughed at their jokes, asked personal questions and kept up an almost continuous conversation.

Callum and Owen received most of her attention. Both were fit young men who managed the path without difficulty.

At lunch, the students sat in small groups at a viewpoint, sharing snacks and comparing field notes about the plants, birds and other wildlife they had seen. Rebecca moved among them but spent most of the break beside Callum, Owen and two of the other men.

"The formations here are incredible," she said as she sat beside Callum.

She leaned over to study his camera screen, pressing her shoulder against his. Callum showed her several photographs, but whenever she moved closer, he made small, careful adjustments to restore the distance between them.

The afternoon was devoted to navigation, filtered water collection and basic first aid. I demonstrated how to use a map and compass together, then divided the students into pairs.

"Always know where you are and how you can return to a recognised path," I said. "Losing your bearings is one of the quickest ways to turn a good day into an emergency."

Rebecca paired herself with Owen for the navigation exercise. They stood shoulder to shoulder over the map. During the water-filtration lesson, she chose Callum as her partner and repeatedly touched his hands while showing him how the components fitted together.

Her behaviour was unprofessional. I recognised that clearly by then, although I still did not consider it alarming. Most of the students seemed at ease with her, but later that afternoon I caught Callum's expression when she placed a hand on his shoulder while he adjusted his rucksack. He looked distinctly uncomfortable.

That evening at the hostel, I led a safety briefing. There were no bears to worry about, but the Cairngorms presented hazards of their own.

"Give red deer plenty of space, especially if an animal is injured, startled or separated from the herd," I explained. "Watch where you place your hands and feet in warm weather because adders may be lying in the heather. Check yourselves for ticks each evening. Most importantly, never underestimate how quickly mountain weather can change."

I also covered river crossings, emergency whistles, the group's satellite communicator and my personal locator beacon. I showed them the first-aid kit and explained what to do if the party became separated.

The students asked sensible questions. Callum was particularly attentive, taking notes and asking about specific emergency scenarios. It always made my work easier when clients listened properly to a safety briefing.

Most of the group went into town for dinner afterwards. I stayed in the common room with maps spread across a table, reviewing the route for the following day.

Rebecca joined me after a while and sat close enough for me to smell her shampoo.

"The students are really responding to you," she said, tracing the route with one finger. "They respect your experience."

"That's good to hear." I shifted slightly away from her. "Respect for the guide makes everyone safer."

"Safety first, always," she said with a smile. "That's what I keep telling them."

She leant forwards to inspect the map, pressing her arm against mine. I moved my chair back a few centimetres, making the movement appear casual. Years of guiding had taught me how to establish professional distance without creating an unnecessary confrontation.

Rebecca appeared to understand and sat back.

Our second training day covered more advanced skills: choosing a safe camping area, protecting the ground, storing food responsibly and signalling in an emergency. We walked to a broad patch of open ground bordered by Scots pine and practised pitching tents and tarps.

"Your tent should be on level, durable ground," I told the group as they examined possible pitches. "Avoid hollows where rain will collect, and don't damage vegetation simply to make yourselves more comfortable."

The students separated into their assigned pairs and began setting up their shelters. Rebecca circulated among them, offering advice. Once again, she positioned herself most often near Callum and Owen. She leant close while asking questions, touched their arms to emphasise points and found reasons to join them during checks.

When we demonstrated tarp rigging, she asked Callum to hold one corner while she secured the other. They worked efficiently, but she repeatedly brushed against him as they adjusted the fabric. Her fingers lingered around his wrist while she demonstrated how to tension a guyline.

After lunch, we rehearsed emergency responses ranging from minor injuries to severe trauma. The students worked in pairs, bandaging simulated wounds and building an improvised stretcher.

"In remote country, you must be ready to help one another," I said. "Mountain rescue may take hours to reach you, especially in poor visibility."

Rebecca volunteered to play the casualty. She lay on the ground while Callum and Owen worked out how to lift her safely. She laughed when they struggled to keep the stretcher level and gripped Callum's arm once they had raised her.

We ended the day with a full review of the expedition route. Back at the hostel, maps covered several tables in the common room.

"Tomorrow we begin the approach to Loch Avon," I said, tracing the line with my finger. "It's approximately nineteen kilometres, with about four hundred and sixty metres of ascent along our planned route. We'll establish a base camp near the loch and spend three days studying the surrounding habitats and landforms before walking out."

The students asked what they might see and what conditions to expect. Their excitement grew as the field course began to feel real.

"Loch Avon is one of the most extraordinary places in the national park," I told them. "The water lies beneath steep mountain walls, and the light can change the whole character of the place from one minute to the next. It's excellent country for photography."

I nodded towards Callum, who smiled.

After confirming our departure time and making the final equipment adjustments, I repeated the emergency procedures and stressed the importance of remaining with the party.

"Out there, we follow three simple rules," I said, looking at each student in turn. "Stay together, communicate clearly and follow safety instructions immediately."

Even the most casual students nodded with appropriate seriousness.

As the meeting ended, some went upstairs to pack while others walked into town for last-minute supplies. I remained behind to check my own equipment and examine the latest weather forecast.

Through the window, I saw Rebecca speaking to Callum beside the minibus. The conversation looked intense. She stood very close, gesturing as she spoke. Callum kept looking towards the hostel as if afraid somebody might see them. His shoulders were tight, his weight angled away from her.

I frowned and went outside.

Rebecca stepped back as soon as she saw me. A bright smile appeared on her face.

"Martin? We were just discussing tomorrow's weather," she said with forced cheerfulness. "Callum was worried about his camera equipment if it rains."

Callum stared at the straps of his rucksack.

"Yes. Just checking about rain covers and things."

"The forecast is clear," I said, looking from one to the other. "But it always pays to be prepared."

Callum muttered that he needed to finish packing and walked quickly into the hostel. Rebecca remained beside me, still smiling as if nothing unusual had happened.

"Everyone's excited about reaching the loch," she said as she fell into step with me. "They're packed and ready for the early start."

I watched Callum disappear into the building. Something about the exchange troubled me, although I could not yet identify exactly what was wrong. I resolved to watch their interactions more carefully once we were on the route.

I slept badly. My thoughts kept returning to the tension in Callum's posture and the false brightness of Rebecca's smile. I had nothing concrete on which to act, but instinct told me I had missed something important.

Morning came early. I was awake before sunrise, checking the forecast and making my final preparations while the students ate breakfast in the dining room.

"Rucksacks in the minibus in fifteen minutes," I called as I passed through. "We've a full day's walk ahead."

They hurried to finish eating. Callum sat alone at a corner table, pushing food around his plate. When Rebecca entered, his shoulders tightened and he immediately stood to clear away his tray.

We loaded the minibus and drove to the start of our approach into the national park. The morning air smelt of damp earth and pine as I helped unload the rucksacks and distributed the communal supplies.

Our group equipment included a large mess and emergency shelter rated to hold the entire party if conditions forced us together. Food and toiletries were sealed in odour-resistant drybags and would be stored away from the sleeping tents. I carried my first-aid kit, a powerful torch, whistle, satellite communicator and personal locator beacon.

The students were eager to begin. Even the less experienced walkers adjusted their rucksacks with confidence after two days of practice.

"Gather round," I said once everyone was ready.

All twelve students and Rebecca formed a circle, tightening straps and checking water bottles.

"We have about nineteen kilometres to cover, with roughly four hundred and sixty metres of ascent. We'll stop regularly, but you must manage your pace. If anything hurts or feels wrong, tell me early. Don't wait until it becomes a problem."

Several hands rose with the usual questions about toilet stops, water and wildlife. I answered each in turn. Callum stood at the back with several students between himself and Rebecca, who had placed herself near the front.

"Let's get moving," I said, shouldering my rucksack. "Stay together and keep an eye on one another. Watch the weather, and take care where the path crosses wet ground or running water."

I led the way, with Rebecca close behind. The mountain path began broad and clear, passing through pine woodland before opening periodically to views of the surrounding slopes. The students stretched into a loose line and talked among themselves as we found a steady rhythm.

"A beautiful morning for it," Rebecca said, matching my pace. "They're thrilled to be heading for the loch at last."

"It's worth the walk," I replied, keeping my tone professional. "One of the finest places in the park."

"I can't wait to see it."

She dropped back to walk beside Owen. Her fingers brushed his arm as she pointed towards a bird circling above the trees.

For the next few hours, I watched her move repeatedly through the line, seldom remaining with one person for more than fifteen minutes. She appeared sociable and attentive, but a pattern was unmistakable. She gravitated towards the male students, especially Callum and Owen, finding reasons to walk beside them, adjust their equipment or share observations.

When Callum noticed her approaching during the middle of the morning, he increased his pace until he reached me at the front.

"Do you mind if I ask a few questions about the area?" he said, glancing over his shoulder.

"Not at all. What would you like to know?"

I recognised his tactic. He asked thoughtful questions about local geology, birdlife and the upland environment, displaying genuine interest while keeping his place beside me. Behind us, Rebecca's expression tightened for an instant before she turned to another male student.

At about midday, we reached a burn crossing the path. Clear water moved over rounded stones, and the bank offered enough firm ground for a lunch stop.

"Thirty minutes," I announced, lowering my rucksack onto a flat rock. "This is a good place to rest and refill your bottles, but use your filters."

The students spread out along the bank. Some unpacked food while others crouched beside the burn with their water filters. I moved among them, checking for fatigue, hot spots and blisters and making sure everybody drank.

I was helping Hannah fit the hose to her filter when Callum approached. He looked nervous and glanced around to make certain nobody was near enough to hear.

"Martin, could I speak to you for a moment?" he asked quietly. "Somewhere private?"

I finished helping Hannah and nodded towards the path upstream.

"Let's check the route ahead."

We followed the burn until a bend concealed us from the group. Callum walked in silence, repeatedly looking back. His shoulders were rigid, and he pushed a hand through his sandy hair.

I stopped where the water broadened into a shallow pool.

"What's troubling you?"

He drew a long breath and stared down at his boots.

"It's Rebecca," he said, almost too softly to hear above the water. "She's been making me really uncomfortable."

My stomach tightened. I had suspected as much, but hearing him say it made the situation immediate and real.

"Tell me what's happened," I said, keeping my voice calm.

"Last night, after everyone had gone to bed, she knocked on my room door. She said she wanted to discuss some biology questions and talk about the trip. Then she started getting personal."

My jaw tightened.

"How personal?"

"She sat beside me and put her hand on my leg. Then she asked whether I had a girlfriend back home. When I told her about Lucy, she smiled and said, 'She's not here, though.'"

Callum ran his hand through his hair again.

"I made an excuse about needing to pack. I practically pushed her out of the room."

"Has she done anything like that before?"

"Little things. Touching my arm, standing too close, finding excuses to put her hands on me. I thought perhaps that was simply how she behaved with everyone. But last night crossed a line. And outside the hostel, she was trying to persuade me not to make a fuss. That's what she was talking about when you came over."

Anger rose in my chest, but I kept it from my voice.

"You did the right thing by telling me. None of that is acceptable. I'll keep you away from her as much as I can, and when we return I'll make certain the university is informed. If she approaches you again, come straight to me."

"Please don't make it obvious," he said quickly. "I still have to take her class next term."

"Your safety matters more than that. But I won't confront her here unless I have to. We're in remote country, and my first responsibility is keeping the group stable and getting everyone back safely. For now, stay close to me or to the others. You won't be left alone with her."

He nodded, though he did not look reassured.

We turned back towards the lunch stop. Through the trees, I could see Rebecca standing among the students, smiling as if nothing at all were wrong.

"Just finishing up," I said, meeting her eyes directly. "Callum was asking about our campsite at Loch Avon."

Rebecca glanced between us. Her smile faltered, but only for a moment.

"It's supposed to be gorgeous. I can't wait to see it."

The drive to the start of the mountain path took just under an hour. The students chatted excitedly in the back of the minibus while I mentally revised our camping arrangements. What Callum had told me was not merely unprofessional. It was predatory.

An academic, even a doctoral teaching assistant, making advances towards an undergraduate violated every ethical standard the university claimed to uphold.

At the trailhead, I gathered everyone for final instructions before we set off towards Loch Avon. The approach was approximately 19 kilometres, with about 460 metres of ascent, and would take most of the day. I wanted them concentrating on the route ahead rather than whatever was happening between Rebecca and Callum.

The walk itself was spectacular. Our path wound through Scots pine and heather, occasionally opening onto immense views of the Cairngorm mountains. Great shoulders of rock rose around us, with streaks of old snow lingering in shaded corries despite the mild September weather. The air was clean and sharp, carrying the scents of pine resin, damp earth and wild grass.

I kept the group moving at a steady pace, stopping regularly for water and rest. At one burn, the students refilled their bottles through their filters while I checked the route and the weather.

Throughout the day, I positioned myself between Callum and Rebecca whenever I could. She seemed to notice. At rest stops, she gave me questioning looks when I appeared beside him or quietly rearranged the walking order before we moved on.

"Everything all right?" she asked during our lunch stop beside a narrow burn. "You seem tense today."

"Just focused on getting everyone to camp before dark," I replied, keeping my tone deliberately neutral.

We reached Loch Avon in the late afternoon. The trees and scrub had long since thinned, and the loch lay ahead of us beneath steep mountain walls, its dark blue surface reflecting the surrounding crags. Even after twenty-two years of guiding in those mountains, the sight still took my breath away.

"Wow," Hannah whispered, stopping in her tracks.

"Worth the walk?" I asked, smiling at the students' stunned expressions.

We reached the shore with about two hours of daylight remaining, enough time to establish camp before darkness fell. I chose a level wild-camping area set back roughly 30 metres from the water, on firm ground that would withstand the night's use without being churned into mud. There was room for the sleeping tents, my separate guide tent and the large communal mess and equipment shelter, which was rated to hold the entire party in an emergency. Our raised, low-impact fire bowl went on a bare patch of stony ground well away from the tents.

"Let's get camp set up," I called as everyone lowered their rucksacks with grateful sighs. "Tents first. Dinner afterwards."

This was where I put into effect the changes I had been considering all day. Normally, the students chose their own tent groups, but I had a responsibility to protect Callum without publicly humiliating him or exposing what he had told me in confidence.

"I'm assigning the sleeping tents," I announced, taking out my notebook. The students looked surprised, but no one objected. "Hannah, Bethan and Rebecca, you're together. Owen, Callum and Ravi, take the tent on the right. The other nine of you, divide yourselves between the remaining tents as we agreed earlier. I'll be in the guide tent across the site, beside the communal shelter."

I watched Rebecca carefully. Her smile slipped before she regained control of it.

"I thought the academic trip leader normally had a tent to herself," she said lightly.

"Not with this arrangement," I replied. "We need everyone close together, and the communal shelter has to remain available for equipment and emergencies."

I half expected her to challenge me. Instead, she shrugged and picked up her rucksack.

"Whatever works."

The students spread out to pitch the tents. While putting up my own, I kept an eye on Rebecca. She worked mechanically alongside Hannah and Bethan, her earlier enthusiasm gone.

Once the shelters were secure, I organised the cooking rota. Three students prepared lentil stew while others collected a modest amount of dead wood for the fire bowl or filtered water from a nearby burn. Rebecca stood near the bowl, watching Callum as he helped Owen rig a cooking tarp against possible rain.

"Rebecca," I called, "could you organise the food bags? We need to separate what we're using tonight from what we'll seal away."

She turned slowly, almost reluctantly dragging her gaze from Callum.

"Sure," she said flatly.

As dinner preparations continued, the change in her became more pronounced. The animated, engaging teaching assistant from the first few days had disappeared. She grew quiet, her movements stiff and distracted.

When one of the students asked where the seasoning for the stew was, Rebecca snapped, "How should I know? Ask Martin."

The student recoiled from her tone.

"It's in the red drybag with the other cooking supplies," I said, giving Rebecca a questioning look.

She ignored me.

Dusk settled over the loch as we gathered near the fire bowl to eat. The mountains became black silhouettes against the fading sky, and the first stars appeared overhead. Under ordinary circumstances, it would have been a perfect evening in the Highlands.

Rebecca barely touched her food. She pushed the stew around her bowl while staring across the fire at Callum. Her concentration was so intense that several students noticed and exchanged uncomfortable glances. Callum kept his eyes on his meal or on whoever happened to be speaking beside him, avoiding Rebecca's gaze completely.

After dinner, I gathered everyone for a short briefing about the following day's fieldwork.

"We'll divide into two groups tomorrow," I said. "One group will follow the loch shore to examine geological features. The other will study plant adaptations higher above the camp."

Rebecca's eyes never left Callum. She was not even pretending to listen.

When I assigned her to lead the alpine ecology work and placed Callum in my geology group, her head snapped towards me.

"I should go with the geology group," she said sharply. "It's more relevant to what the students are studying this term."

"Your specialism is alpine ecology, isn't it?" I asked, although I knew perfectly well that it was. She had told us during her introduction. "The students will benefit more from your expertise there."

Her jaw tightened, but she did not argue further.

As the meeting ended and the students drifted towards their tents or remained by the fire bowl, I drew Callum aside.

"Stay with the group tonight," I said quietly. "Don't go anywhere alone."

He nodded, understanding without further explanation.

"Thanks for changing the groups tomorrow."

"No problem. Try to get some rest."

While the students cleaned the cooking equipment, I prepared our supplies for the night. All food, cookware and toiletries went into odour-resistant drybags, which we stored together away from the sleeping area in accordance with leave-no-trace practice.

I carried the last of the bags about 60 metres from camp. Beyond the reach of my head torch, the darkness was almost absolute, broken only by thin patches of moonlight. As I secured the drybags, I heard something moving among the heather and scattered trees beyond me.

I stopped and listened.

Twenty-two years in the mountains had taught me to distinguish the tread of red deer from the lighter movements of foxes and smaller animals. This was different. It was not the cautious progress of an animal feeding, but something erratic and violent. Twigs snapped. Then came a furious scraping sound, followed by a low, rumbling growl.

My first thought was an injured red deer, or perhaps a poacher's dog loose on the hillside. I took the powerful torch from my belt, gripped one of my trekking poles and turned the beam to full strength.

"Who's there?" I called firmly.

The sound stopped for a moment, then resumed with greater force. The growling became louder, broken by harsh grunts.

I moved cautiously towards it, making my presence obvious with heavy steps and repeated calls. Roughly 45 metres from where I had left the drybags, my torch beam found movement on the ground.

I stopped dead.

Rebecca was on her hands and knees, digging frantically into the soil. Her fingers tore through earth, roots and pine needles with manic force, flinging debris behind her like a dog excavating a burrow.

The growling was coming from her.

"Rebecca," I called, confusion giving way to a creeping dread.

She froze at the sound of my voice but did not turn. Her back rose and fell with rapid breaths.

I stepped closer and directed the torch fully onto her.

"Rebecca, what are you doing out here?"

When she finally turned, her eyes reflected the beam with an animal brightness. Her face was smeared with soil and something dark and wet. Blood covered her mouth and chin and dripped onto the front of her clothes.

"Are you hurt?" I asked, moving towards her out of instinct.

She did not answer. A guttural noise rose from deep in her throat, raising the hairs along the back of my neck. I had never heard a human being make a sound like it.

"Rebecca," I said more firmly. "What's wrong? Talk to me."

Her head tilted at an unnatural angle. She studied me as a predator might assess prey.

Then she lunged.

I stumbled backwards as her teeth snapped shut where my forearm had been a second earlier. She moved with shocking, impossible speed. Landing on all fours, she immediately sprang up and charged again.

I dropped the torch in my panic and scrambled backwards, tripping over a fallen branch. Rebecca missed me by centimetres. Her teeth clacked audibly on empty air before she crashed into the undergrowth and vanished beyond the fallen beam of light.

I remained frozen, breathing in short gasps and straining to hear where she had gone.

The mountainside had fallen silent. There was no movement and no growling, only the faint stirring of pine needles and heather in the night breeze.

My mind refused to accept what I had seen. This was not a mental health crisis, at least not like any I had ever encountered or heard described. Rebecca had moved and sounded like a wild animal, something hunting rather than reasoning.

The image of her dirt-streaked face and shining eyes stayed before me as I retrieved the torch and hurried back towards camp. I went as quickly as the uneven ground allowed, stumbling twice over roots and stones hidden in shadow. The beam bounced wildly ahead of me, throwing disorienting patterns across the trees.

Twenty-two years in the Scottish Highlands had taught me to trust my instincts, and every instinct I possessed was screaming danger.

The glow from the fire bowl appeared ahead. Most of the students had gone to their tents, though a few remained beside the dying flames. I slowed as I approached, unwilling to start a panic until I had some sort of plan.

We needed to leave, but leading twelve undergraduates through difficult mountain terrain in darkness would be dangerous. The staffed ranger base was about 13 kilometres away, and the satellite communicator and personal locator beacon were in my separate tent on the far side of the campsite.

I saw Callum and Owen still awake, talking quietly beside the fire bowl. I went straight to them and crouched between them, glancing towards the darkness behind me.

"We need to leave. Now."

Callum became alert at once.

"What's wrong?"

"It's Rebecca. Something's happened to her." I struggled for words that would not sound absurd. "She attacked me out there."

Owen's eyes widened.

"Attacked you? What do you mean?"

"There's no time to explain. We need to wake everyone and keep the group together. Help me get-"

A twig snapped at the edge of the campsite.

All three of us turned.

Rebecca stood where the firelight met the darkness. She bore no resemblance to the composed teaching assistant who had arrived with the students. Her clothes were torn and filthy, and soil and pine needles were tangled through her hair. Both hands were caked with earth and blood. More blood dripped from her mouth and chin, glistening in the light.

"Oh my God," Owen whispered.

The most disturbing thing was the way she moved. Her head jerked in rapid little motions as her eyes darted between us. Her body twitched and spasmed, as though she were fighting against her own muscles.

"Rebecca?" Callum called, slowly rising. "Are you hurt?"

She fixed on him instantly. Her head snapped towards him with a speed too abrupt and mechanical to be human. That same deep growl came from her throat.

"Back away slowly," I said, reaching towards the fire bowl. "No sudden movements."

Callum took one cautious step backwards.

Rebecca's body tensed.

What followed happened too quickly for me to stop. She hurled herself forwards, covering the distance between them in an instant. She struck Callum hard enough to knock him flat, straddled his chest and drove her teeth into his shoulder.

His scream tore through the night.

I seized a burning length of wood from the raised fire bowl, ignoring the heat against my palm, and swung it into Rebecca's back. It struck with a heavy thud. Sparks scattered as she released Callum with a shriek more animal than human.

She spun towards me, her bloodied face twisted with rage and her teeth bared.

"Get Callum!" I shouted, holding the burning wood between us.

Owen rushed forwards and tried to pull Callum upright. Blood seeped between Callum's fingers as he clutched his shoulder. Something darker and thicker was mixed with it, a viscous black fluid that appeared to be leaking from the bite itself.

"Come on, mate," Owen said. "Up you get."

The movement caught Rebecca's attention. She whirled away from me as though I had ceased to exist and launched herself at Owen.

I swung again and missed.

She tackled Owen to the ground with the same appalling strength. Before I could reach them, her teeth sank into his neck.

Owen's scream woke the entire camp. Students burst from the sleeping tents, their confusion turning to terror as they saw what was happening. Some froze. Others screamed. Within seconds, the peace of the mountain night had become chaos.

I dropped the burning wood and caught Rebecca from behind, locking my arms around her torso and hauling her off Owen. She fought with strength that should have been impossible for someone her size, twisting violently and snapping her teeth at my arms.

"Get to the communal shelter!" I shouted at the students. "All of you, now!"

Owen lay on the ground with one hand pressed to his neck. Even in the uncertain firelight, I could see the black substance from Rebecca's mouth mixing with his blood and spreading outwards from the wound like ink through water. Darkness travelled beneath his skin, tracing the veins of his throat.

Rebecca writhed in my grip. Her spine bent at angles that made my stomach turn. Her fingernails, jagged and packed with soil, raked across my forearm and opened deep gashes.

"Help me," Callum called weakly from beside the fire bowl.

He had managed to sit up, but his face was deathly pale. When he pulled his hand from his shoulder, I saw black veins spreading from the bite and disappearing beneath his torn shirt.

Whatever this was, it was advancing before my eyes.

Rebecca suddenly went limp. The change caught me off guard. As my grip slackened, she dropped to all fours and scuttled away with the same broken, unnatural motion.

She began circling at the boundary of the firelight, her head twitching as she surveyed the camp. Several students still stood paralysed near their tents.

"Move!" I shouted, pointing towards the large communal shelter. "Everyone inside!"

Two students ran to Callum. I went to Owen. His eyes were open but unfocused, and his skin was burning when I touched his forehead. The black veins had already spread from his neck across his collarbone and upwards towards his jaw.

"Owen, can you hear me?"

He muttered something incoherent. His eyes rolled back.

I hooked my arms beneath his shoulders and dragged him towards the shelter. Pain shot through the wounds in my forearm, but the adrenaline kept me moving.

Rebecca paced beyond the firelight, darting forwards every few seconds and then retreating. She reminded me of a predator testing a herd, searching for the weakest point.

"Martin!" someone screamed. "She's coming!"

Rebecca changed direction and charged straight at us. I could not abandon Owen, and the shelter was still several metres away.

Ravi snatched up a heavy cooking pot and hurled it. It struck Rebecca's shoulder, throwing her off balance long enough for me to drag Owen the remaining distance.

"Take him!" I shouted.

Several students reached from the entrance, caught Owen beneath the arms and pulled him inside.

I ran back two paces and snatched up the burning wood. Its flames had diminished, but one end still glowed red. Rebecca had recovered and was watching me with those reflective eyes.

"Stay back!"

I swung it in a broad arc. She snarled, exposing teeth stained black with the viscous fluid.

For a moment we faced one another, me holding a length of smouldering wood, Rebecca possessed by strength and speed I could not understand.

"Martin, hurry!" Callum called weakly from inside.

I backed towards the entrance. Rebecca paced with me, occasionally feinting forwards and withdrawing whenever I swung. There was calculation in her movements. She was waiting for a mistake.

When I reached the shelter, I felt behind me for the opening without taking my eyes from her.

"I'm coming in. Be ready to close it immediately."

As I backed through, Rebecca charged.

I thrust the glowing end towards her face. She veered away with a shriek, and I threw myself into the shelter. The students hauled me clear and zipped the entrance behind me.

The ten uninjured students crowded together under the beams of several head torches. Callum sat propped against rucksacks, his face grey, while Owen lay unconscious on the ground, breathing in rapid, shallow gasps. With the two injured students, all twelve undergraduates were accounted for. Rebecca remained outside, and I was the only person between the group and whatever she had become.

"What's happening?" Hannah asked, her voice breaking. "What's wrong with Rebecca?"

"I don't know," I admitted.

I knelt beside Callum. The bite looked worse at close range. The flesh surrounding it was swollen and angry red, and black veins radiated from the punctures like cracks in glass. He flinched when I touched the skin beside the wound.

"It burns," he said through clenched teeth. "Like there's fire under my skin."

Rebecca moved around the shelter outside, growling as her feet scraped over the ground. Something dragged across the fabric. It might have been her nails or her teeth.

"Barricade the entrance," I said. "Use rucksacks, sleeping bags, equipment cases, anything heavy."

The students obeyed at once, piling supplies against the zipped doorway while I examined Owen.

His condition had deteriorated within minutes. Dark veins covered most of his neck and had spread across his cheek and forehead. His skin burned beneath my hand, and his breathing was irregular and laboured.

"Martin," Callum said weakly, "what's happening to us?"

I looked from the infection spreading through his shoulder to Owen's unconscious body. Nothing in my mountain first-aid training had prepared me for this. No ordinary infection moved so rapidly or changed a person so completely.

"I don't know," I said. "But we're going to get both of you help."

"It's inside me." Callum stared at the black veins creeping down his arm. "I can feel it moving."

A scratching noise came from the side of the shelter. Rebecca's shadow showed through the fabric, hunched and distorted as she circled us.

"She's testing for weak points," I said quietly. "Looking for a way in."

The students pressed closer together, keeping as far from the walls as they could. Someone cried softly in the darkness.

"We need to call for help," Bethan said, her voice trembling. "What about the satellite communicator? Or your beacon?"

"They're in my tent," I replied. "Across the campsite."

I searched my pockets for anything useful. All I found was a folding multitool, my head torch and a partly filled water bottle. I gave the bottle to Callum. He managed a small sip before passing it back.

"How far is the ranger base?" Ravi asked.

"About 13 kilometres, through difficult terrain. We'd have to cover it in darkness."

"That's better than staying here."

"Not with Rebecca outside," I said. "And we can't leave Callum and Owen behind."

Owen groaned. His back arched from the ground, his eyes moving beneath their closed lids as his fingers clawed at the floor.

"What's happening to him?" someone whispered.

"Whatever is in the bite is spreading," I said, although I had no real understanding of what we were facing.

For the next hour, we sat in taut silence and listened to Rebecca circle the shelter. At intervals, she clawed the fabric or flung her body against it, making the students gasp and cry out. Each time, she paused as if measuring our fear before resuming her patrol.

I checked my watch. It was just after 23:00. The night stretched ahead of us, long and uncertain.

Owen worsened by the minute. The black veins spread across his entire face and down his chest. His breathing became harsher, and violent twitches ran through his body.

Callum was scarcely better. The discolouration from his shoulder had travelled down his arm and across his chest. He remained conscious, but his strength was fading and his eyes were glazed with pain.

"My head," he muttered. "Everything's too loud. Too bright."

I tore strips from a clean T-shirt and bound the bite as well as I could. The fabric blackened almost immediately, soaked by the strange fluid seeping from the wound.

"This doesn't make sense," Hannah whispered. She was studying biology, if I remembered correctly. "Infections don't spread this fast."

"This isn't a normal infection," I said, securing the makeshift dressing.

"Then what is it?"

I had no answer that would not have sounded insane.

Around midnight, Owen began to convulse. His back arched off the ground, his limbs rigid and shaking. I rolled him onto his side and protected his head, but there was little else I could do.

The seizure lasted almost a minute. Then he became completely still.

I felt his neck for a pulse.

There was nothing.

"Is he...?" Hannah could not finish.

I nodded.

"He's gone."

Quiet sobbing filled the shelter.

Outside, Rebecca's growling intensified, as if she somehow knew what had happened. I covered Owen with a sleeping bag, both for dignity and to spare the others from looking at him.

Callum watched in horror. He must have seen his own approaching death beneath that cover.

"That's going to happen to me, isn't it?" he whispered.

Before I could answer, Rebecca's growls changed pitch. She moved faster around the shelter, her steps and scraping becoming increasingly frantic.

"Why is she getting worse?" Bethan asked, pressing herself against the far wall.

"I don't know."

But a terrible suspicion had begun to form.

The minutes crawled past, marked by Rebecca's growling and the ragged breathing of the terrified students. Callum continued to deteriorate, the black veins climbing his neck towards his face.

Twenty minutes after Owen died, his body jerked beneath the sleeping bag.

A student screamed. Everyone recoiled.

"He moved!" Ravi shouted. "I saw him move!"

"That's impossible. He has no pulse."

I reached for the sleeping bag and pulled it back.

Owen's eyes snapped open.

They were bloodshot and possessed the same unnatural reflective quality I had seen in Rebecca's. Before I could react, he sat upright with a violent, disjointed motion no living person would make.

"Get back!" I shouted, shoving the nearest students away.

Owen's head twitched as he surveyed the shelter. His movements were identical to Rebecca's: jerking, mechanical and inhuman. When his gaze settled on Ravi, his lips drew back in a snarl. His teeth were already stained with the same black fluid.

The truth came to me with sickening clarity. Whatever had changed Rebecca had passed into Owen through her bite. He had died and returned as something else, something no longer human.

Callum stared at him with horrified recognition. The same thing was already moving through his body, only more slowly.

I opened the blade on my folding multitool and gripped it tightly as Owen began to move. The students crowded against the far side of the shelter, desperate to put as much distance as possible between themselves and him.

Outside, Rebecca's frenzied growls mingled with Owen's. It was as though she could sense that something like her had awakened inside with us.

I placed myself between Owen and the students, the small blade raised, knowing I might have to make an impossible choice to protect them.

Owen's muscles tensed. Rebecca's growls rose in pitch, excited and urgent.

Then Owen lunged at Ravi, teeth bared and fingers curled like claws.

I threw myself between them and tackled him with all my strength. We crashed into the wall of the shelter, making the entire structure sway as we rolled across sleeping bags and rucksacks. The students screamed and scrambled away from our struggle.

Owen was impossibly strong.

His hands clawed at my jacket, trying to reach bare skin. I fought to pin his arms, but Owen twisted beneath me with unnatural flexibility. His teeth snapped inches from my face, spraying black fluid as he snarled.

We rolled again and struck the shelter's central support pole. Owen's head whipped back against the metal with a hollow crack that echoed through the fabric. His body went limp beneath me.

I held him down for several seconds, waiting for him to move again. He did not.

Blood pooled beneath his head, mixing with the black substance still leaking from his mouth.

"Is he...?" Hannah whispered from the far side of the shelter.

I checked for a pulse and found nothing.

"He's dead."

Outside, the sounds had stopped. Rebecca's growls and scratching, constant for hours, had vanished into the night. The sudden silence felt almost as threatening as her presence had. I listened intently, barely breathing as I tried to detect movement beyond the shelter walls.

Nothing.

"She's gone," I said, though I could not be certain. "We need to move while we have the chance."

"Move where?" Bethan asked, her voice shaking. "It's the middle of the night."

"To the mountain ranger base. It's about 13 kilometres away." I glanced at Callum, who was slumped against a pile of rucksacks, barely conscious. The black veins had spread up his neck and across his jaw. "We have an injured person who needs urgent medical attention."

"What about Owen?" someone asked.

I looked at his body, knowing we could not take him with us.

"We have to leave him. The authorities will recover his body later."

The words sounded callous, but I had no choice. Our priority had to be the living.

"Take only what you absolutely need," I instructed. "Water, some food and an extra layer. Nothing else. We travel light and fast."

I moved to Callum's side and gently shook his shoulder.

"Callum, can you hear me?"

His eyes fluttered open, glazed with pain and fever.

"Martin?"

"We're leaving. Can you walk if I help you?"

He nodded weakly. "I think so."

"What about her?" Hannah asked, glancing towards the shelter wall.

"I'll check. Everyone stay quiet and be ready to move the moment I say it is clear."

I opened the entrance just enough to peer outside. The wild-camping area was eerily still, lit only by the fading embers in our raised fire bowl. I swept my gaze slowly across the tents, the heather and the dark ranks of Scots pine beyond them.

Nothing moved.

"I can't see her," I whispered. "Get ready."

The ten uninjured students gathered their few essential belongings while I helped Callum to his feet. He swayed dangerously and leant most of his weight against me.

"I've got you," I said, wrapping an arm around his waist.

"My shoulder," he mumbled. "It feels as if it's on fire."

I opened the shelter fully and stepped out, supporting him with one arm while gripping my folding multitool in the other hand. It was a poor weapon, but it was all I had. The pre-dawn air was cold and still, carrying the scent of pine, peat and something metallic, almost rotten, that had not been there before.

"Come on," I called softly to the students inside. "Quickly, but quietly."

They emerged one at a time, their eyes wide as they searched the darkness. Hannah helped Bethan, who was still crying under her breath. Ravi moved towards the front, his head torch cutting a narrow path through the gloom.

"Don't go too far ahead," I warned him. "We stay together."

Callum stumbled beside me, every step a visible effort.

"I can make it," he insisted when he saw my concern.

I tore a strip from my shirt and rebound his wound. Black fluid was already seeping through the previous dressing. When I wiped some of it away, I could see Rebecca's bite marks clearly. They were swollen and inflamed, and the surrounding skin had turned an unnatural grey.

"We need to keep this clean," I said, although I suspected that nothing I did would stop what was happening inside him.

We left the campsite in a tight group, our head torches illuminating the mountain path that would lead us back towards help. No one spoke. Every snapping twig and every rustle in the heather made us stop and listen for pursuit.

Callum's condition worsened with every kilometre. His skin burnt with fever and he leant more heavily against me. The black veins spread farther across his face, becoming visible along the edge of his jaw. His breathing grew laboured, each breath a short, painful gasp.

"I need to rest," he whispered after we had covered roughly three kilometres.

I called a brief halt and helped him sit on a low boulder beside the path while the others gathered around us. Dawn was breaking, pale light seeping over the corries and through the trees. Better visibility made me feel marginally safer, though I continued searching the slopes for Rebecca.

"Five minutes," I told the group. "Drink some water. Eat something if you can."

Most of them stared at me blankly, too shocked or exhausted to feel hungry. Hannah came closer and lowered her voice.

"What was wrong with Rebecca? And Owen? People don't simply change like that."

"I don't know," I said. "I've never seen anything like it."

"It's spreading through Callum as well. Those black veins are the same as Owen's."

I nodded grimly. "Which is why we have to get him to hospital as quickly as possible."

Callum was watching us. His eyes were clearer than they had been before.

"I heard that," he said weakly. "Am I going to die like Owen did?"

"No," I said firmly. "We're getting you help."

But Callum had seen Owen die and then rise as something else. The fear in his eyes told me that he understood precisely what might be happening to him.

After the short rest, we continued. The path grew steeper as we climbed out of the Loch Avon basin. In ordinary circumstances it would have been a demanding but manageable walk. With ten exhausted and traumatised students, and Callum barely able to stand, our progress was painfully slow.

The sun rose, burning away the mist. Birds began to call among the trees, a surreal contrast to the horror we had escaped. The world carried on as if nothing had happened, unaware of how completely it had changed for us overnight.

By mid-morning, Callum could no longer bear his own weight. Ravi and I took turns supporting him between us. His skin was fiercely hot. He drifted in and out of consciousness, mumbling incoherently whenever he surfaced.

"A little farther," I told everyone as we crested a ridge. "The ranger base should be visible soon."

Hannah had taken charge of the other students, keeping them moving whenever they wanted to stop. She stayed at the back of the line and made certain that no one fell behind. I was grateful for her help. I could not have managed the whole group alone.

At last, through a break in the trees, we saw the ranger base: a small timber building with a communications mast in a clearing less than a kilometre ahead. The sight of it gave everyone a final burst of energy.

"We made it," Bethan said, tears of relief running down her face.

We covered the remaining distance as quickly as we could and stumbled through the door with the last of our strength. The lone ranger on duty sprang to his feet, shocked by the sudden arrival of Martin Hale, ten filthy and exhausted students, and an eleventh student hanging unconscious between Ravi and me.

"What happened?" he demanded, moving at once to help us lower Callum on to a bench.

"We were attacked near Loch Avon," I said, keeping to the simplified account I had already decided to give. "One student is dead and our academic trip leader is missing. This student needs immediate medical treatment."

The ranger responded with brisk efficiency and did not waste time on unnecessary questions. He went directly to the radio and called for an emergency evacuation, contacting Police Scotland, Cairngorm Mountain Rescue Team and the Scottish Ambulance Service.

I collapsed into a chair as the adrenaline that had carried me through the night finally drained away. While he made the calls, I cleaned Callum's wound again and used proper antiseptic from the base's first-aid supplies. The black veins had spread across his shoulder and chest as far as his collarbone. The skin around the bite was mottled grey and black.

"An HM Coastguard helicopter is on its way," the ranger announced. "No more than two hours. Now tell me exactly what happened."

I gave him the edited version. Rebecca Voss had suffered some kind of psychological breakdown, become violent and attacked the students before disappearing into the mountains. Owen Price had been killed during the assault.

I left out the black fluid, Owen's transformation after death, and the inhuman strength and speed that he and Rebecca had displayed.

The ranger took notes and asked for Rebecca's description and the precise location of our campsite. When he asked what might have triggered her breakdown, I could only shake my head.

"She seemed all right until yesterday evening," I said. "Then everything changed."

I gave him the campsite's grid reference and GPS coordinates. Callum stirred on the bench, his eyes fluttering open.

"Where...?" he began, his voice barely audible.

"We're at the ranger base," I told him, kneeling beside him. "Help is coming."

He seized my wrist with surprising strength and pulled me closer.

"Something's wrong with me," he whispered. "I can feel it changing me."

"The doctors will work it out. Just hold on."

His eyes were lucid now, fixed intently on mine.

"If I start to... if I become like them, you have to..."

He did not finish, but I understood. I gave the smallest nod, although I had no intention of doing what he was asking. Apparently satisfied, he released my wrist and closed his eyes.

The other students sat around the ranger base in exhausted silence. Some wept quietly; others stared at nothing. The ranger handed out water and emergency ration bars, but few of them could eat. He remained at the radio, updating the incoming emergency crews on Callum's deteriorating condition.

"Police Scotland will need statements from all of you," he told us, "but that can wait until you've been medically assessed."

I nodded gratefully. None of the students was in any state to be questioned.

Roughly two hours later, we heard the distant thump of rotor blades. Everyone went outside to watch an HM Coastguard search-and-rescue helicopter descend into the clearing beside the base. Scottish Ambulance Service paramedics came running towards us with medical bags and a stretcher.

They went straight to Callum, checking his vital signs and examining the bite. Their expressions tightened when they saw how far the infection had spread.

"How long ago was he bitten?" one of them asked.

"Between fourteen and sixteen hours," I said.

The paramedic exchanged a look with her colleague. "And it has spread this far already?"

"It has been extremely aggressive."

They placed Callum on the stretcher and inserted an intravenous line into his arm.

"We're taking him directly to an isolation room at Raigmore Hospital," the lead paramedic told me. "Whatever this is, it isn't like any wound infection I've seen."

The helicopter evacuated Callum first. It later returned for a second lift, supported by the paramedics and mountain rescue personnel who had reached the ranger base. The remaining ten students and I were assessed, treated for shock and minor injuries, and taken out in groups.

I was among the last to leave. Before boarding, I looked back at the mountain wilderness. Somewhere beyond those slopes, Rebecca might still have been prowling, transformed into something that was no longer human.

As the helicopter lifted and turned north towards Inverness, I stared through the window at the Cairngorms falling away beneath us. Callum was already far ahead, being flown at speed towards Raigmore. I wondered whether the doctors could stop the infection before it completed whatever transformation had taken Rebecca and Owen.

The students sat in silence, each isolated within their own thoughts. Hannah caught my eye, a question in her expression that I could not answer. How could anyone explain something that defied explanation?

At Raigmore Hospital, medical staff were waiting with wheelchairs and trolleys. They hurried to assess us as we arrived. Callum had already been taken into an isolation room, where doctors wearing full protective equipment surrounded his bed.

The rest of us were separated into examination rooms. A doctor thoroughly cleaned and dressed the claw wounds Rebecca had left on my arm while asking me about the incident. I repeated the same sanitised account I had given the ranger.

"And the student in isolation?" she asked. "What happened to him?"

"He was bitten during the attack. The wound became infected very quickly."

She nodded and entered the information in my notes. Scratches alone had not transformed anyone, but none of us understood that distinction with certainty then. The hospital treated my wounds aggressively, cleaning them down to every broken edge of skin and monitoring me for any sign that the black infection had taken hold.

Detectives from Police Scotland were waiting once my examination was complete. They took me into a small interview room, where I gave my statement again, answering as truthfully as I could without mentioning anything that would sound impossible.

"Search teams are heading to the campsite now," the senior officer told me. "They will recover Mr Price's body and look for Ms Voss."

"She's dangerous," I warned. "If you find her, approach with extreme caution."

The officer gave me a measured look. "We're trained to deal with violent and unstable people, Mr Hale."

Not like this, I thought. Not like what Rebecca had become.

After the interview, I was allowed to check on the students. They had been placed together in a ward and were receiving treatment for shock and minor injuries. Their families were travelling north from Brighton and elsewhere in southern England, and the University of Sussex was sending representatives.

A doctor stopped me before I could ask about Callum.

"The student with the infected bite is in isolation," she explained. "We're running tests to identify the pathogen, but it is highly unusual."

"Will he recover?"

She hesitated. "It is too early to say. The infection is advancing rapidly, and he is displaying symptoms we haven't encountered before: an elevated heart rate, extreme fever, and acute sensitivity to light and sound."

They were the same symptoms Callum had described before Owen transformed.

"Can I see him?"

"I'm afraid not. Strict isolation protocols are in force." She touched my arm gently. "Try to rest, Mr Hale. There is nothing more you can do at present."

As I walked away, I knew there was one thing I could do. I could tell them the whole truth. I could describe the black fluid, the transformation after death, and the impossible strength and speed. I could warn them about what Callum might become.

They would dismiss me as traumatised or delusional. They might sedate me or order a psychiatric assessment, and no one would be properly prepared for what was happening to him.

So I remained silent, watching from a distance as clinicians in protective suits entered and left the isolation area where Callum fought a transformation I could not begin to understand.

Police Scotland returned that evening with news.

"We've recovered Mr Price's body," the detective told me. "Search teams are still looking for Ms Voss."

"Did you notice anything unusual about Owen's body?" I asked carefully.

"That isn't information I can share at this stage. The forensic pathologist will conduct a full examination."

I nodded. They would discover soon enough that Owen was not an ordinary corpse. I could only hope they took adequate precautions.

As night fell, I sat alone in the hospital dining room, picking at food I could not taste. My thoughts kept circling back to the transformations we had witnessed: Rebecca digging through the earth like an animal, Owen dying and rising again, Callum changing slowly before our eyes.

Something had infected them. In twenty-two years of mountain guiding, I had never encountered anything remotely like it. Now it might be spreading through Callum's body in an isolation room only a few floors above me, and I had no idea how to stop it.

Three days passed in a blur of hospital rooms, police interviews and broken sleep interrupted by nightmares.

I woke before dawn on the fourth day, disorientated in the sterile room where the doctors had kept me under observation. I had protested that I was physically well, but the wounds on my arm required monitoring and the staff were concerned about my mental state after I had witnessed a student's death. My mobile vibrated on the bedside cabinet.

The screen showed an unfamiliar number.

"Martin Hale."

"Mr Hale, this is Detective Sergeant Morag Fraser of Police Scotland."

She sounded exhausted, as though she had not slept in days.

"I'm calling to tell you that we've located the missing woman from your party."

My grip tightened around the phone. "Rebecca Voss?"

"Yes. A search team found her body early this morning, about 19 kilometres from your campsite."

The word "body" struck me like a physical blow. Despite everything I had seen her do, despite knowing that she had ceased to be human in any meaningful sense, the news of her death shocked me.

"What condition was she in? How did she die?"

The line was quiet for a moment.

"I'm not authorised to discuss those details, Mr Hale. The case has been transferred to UK national public-health and security authorities."

"National authorities? Why would they...?"

"Mr Hale," she interrupted, "I'm calling as a courtesy to let you know that she was found. That's all I can share. I should also advise you not to discuss the incident with anyone, particularly the press."

"I understand," I said, although I did not.

"What about Callum Reed?"

"He remains in hospital and is stable. That is all I know. Someone will visit you later today to take a final statement."

After she ended the call, I sat on the edge of the bed and tried to absorb what I had heard. Rebecca had crossed 19 kilometres of punishing mountain country after leaving the campsite. What had driven her so far? Why were officials refusing to discuss the state of her body?

I dressed slowly, stiff after days of inactivity. Beyond my window, Inverness looked impossibly ordinary in the early light, as though the world had not tilted on its axis four nights earlier beside Loch Avon.

A nurse arrived with breakfast I did not want and the news that a doctor would assess me for discharge later that morning.

"How are the students?" I asked.

"Most were discharged yesterday. Only you and the young man with the infection are still here."

"Callum Reed. How is he?"

She glanced up from her tablet. "I don't work in that unit, but I've heard that he has stabilised."

"Can I see him?"

"You'll need to ask his consultant. Dr Imran Qureshi should be here at about 10:00."

When Dr Qureshi arrived, he was accompanied by a woman in a dark suit who introduced herself as Ms Eleanor Shaw from the UK Health Security Agency. Her identification appeared genuine, but something about her controlled manner made me suspect that public health was not her only remit.

Dr Qureshi examined the healing claw wounds while Ms Shaw watched from the corner.

"These are healing well," he said. "There are no signs of infection. How are you sleeping, Mr Hale?"

"Fine," I lied.

He entered a note on his tablet. "Any continuing anxiety or flashbacks?"

"I'm a mountain guide, doctor. I've dealt with emergencies before."

"Not like this one," he said quietly.

I noticed Ms Shaw adjust her position.

"I'm processing it," I said. "I'd do that better at home."

Dr Qureshi looked at Ms Shaw, who gave a slight nod.

"I can discharge you today," he said, "but I recommend that you speak to a trauma counsellor. What you experienced was severe."

"I know. I'll arrange something when I get home."

I wanted only to leave the sterile rooms and the endless questions.

Ms Shaw stepped forward. "Before you go, Mr Hale, there are some administrative matters to settle."

Dr Qureshi excused himself, leaving us alone. She sat beside the bed and opened a slim case.

"I understand you've been informed about Ms Voss."

"Only that her body was found."

She nodded. "The investigation is continuing, but the official account will classify the incident as an acute psychological breakdown resulting in a violent assault and fatalities. The initial public report will remain deliberately limited while the circumstances of both deaths are reviewed."

"That isn't what happened."

"Ms Voss's death will be attributed to exposure and injuries sustained after she fled into the mountains," she continued, as though I had not spoken. "The University of Sussex has arranged for the surviving students to complete the module through alternative coursework. Your payment for leading the expedition, together with additional compensation, will be transferred electronically today."

She placed a document on the bed.

"There is also a confidentiality agreement requiring your signature. It states that you will not discuss specific details of the trip with anyone who is not directly involved in the investigation."

I stared at it. "You want me to sign an agreement concealing what really happened out there?"

Her expression remained neutral. "It concerns the incident as described in the official report."

"And if I refuse?"

"That would complicate matters for everyone involved, particularly the students who are trying to recover from this tragedy."

She placed a pen on the document.

I thought about Callum, still in isolation with that darkness spreading through him. I thought of the ten other surviving students, all of whom had seen things no one would believe, and of Owen's family, who deserved the truth about their son's death.

But what truth could I offer them? That their son had been infected by something unknown, died and returned as an inhuman predator? That Rebecca had transformed into a creature of impossible strength and speed?

No one would believe me. They would call me traumatised or delusional. I would lose my professional standing, my livelihood and every shred of credibility I possessed.

I took the pen and signed.

Ms Shaw collected the agreement with a small, restrained smile.

"Thank you for your cooperation, Mr Hale. A car will take you back to your cottage when you're ready."

After she left, I sat staring at the payment notification on my phone. The money felt heavier for being invisible, burdened with my complicity in whatever cover-up had closed around us.

I was discharged that afternoon. Before leaving, I made one final attempt to see Callum. I went to the isolation floor, but a nurse at the station stopped me.

"I'm sorry, Mr Hale. Mr Reed isn't permitted visitors."

"I was his guide. I only want to see how he is before I leave."

Her expression softened. "I understand, but those are the consultant's instructions. He is stable, if that helps."

It did not. I needed to see with my own eyes what was happening to him. Had the infection stopped spreading? Had they found a treatment?

"Could you at least give him a message?"

She hesitated, then nodded.

"Tell him Martin came to see him. Tell him... tell him to stay strong."

The official car took me back to my small timber cottage near Grantown-on-Spey late that evening. Everything was exactly as I had left it five days earlier: the mug on my desk, papers spread beside my reading chair, and firewood stacked neatly by the stove. It felt like walking into someone else's life.

I lit the stove and sat watching the flames while the events at Loch Avon replayed in my mind. Rebecca digging in the earth like an animal. Owen's lifeless body reanimating in a series of jerking, inhuman movements. Callum's skin blazing with fever as black veins spread from the bite.

Had I truly seen those things, or had terror distorted my perceptions?

I searched online for news of the incident. There was remarkably little: a brief report of an emergency in Cairngorms National Park involving two deaths, updated after Rebecca's body was found but vague about names and circumstances. It read as if every meaningful detail had been deliberately removed.

The following morning, I rang the University of Sussex. The departmental administrator confirmed that the surviving students had returned safely to Brighton or to their family homes, but would not connect me with any of them, citing their recovery from the tragedy.

When I asked specifically about Callum Reed, she said only that he would not be attending classes for the remainder of the term, then brought the call to a swift end.

Weeks passed. I cancelled the rest of my autumn guiding commitments and told clients that I needed time away for personal reasons. I could not face taking another group into the mountains. Not yet. Not until I understood what had happened.

I made discreet enquiries about the official investigation through contacts in the national park, mountain rescue and Police Scotland. No one could share any details. The case had been sealed, an extraordinary measure for what was publicly described as a field-trip tragedy and violent assault.

October gave way to November. Snow began to dust the wooded slopes around the cottage and settle on the Cairngorm summits.

Small noises woke me at night. When branches scraped over the roof in the wind, my hand went instinctively towards the multitool on the bedside cabinet. I told myself I was being paranoid. Rebecca and Owen were dead. Whatever had infected them had surely been contained.

But I could not forget Callum's face as the black veins spread across his skin, or the sudden clarity in his eyes when he caught my wrist at the ranger base.

If I start to... if I become like them, you have to...

I had not allowed him to finish. I had not wanted to hear what he was asking me to do.

December brought heavy snow and bitter cold. I worked mechanically through my winter preparations, checking supplies, stacking more firewood and making arrangements to close the cottage. I usually spent part of the winter doing seasonal walking work in Dartmoor and the Brecon Beacons. The change of landscape had always cleared my head after a long season.

That year, I doubted anything would help.

Whenever I opened the filing cabinet, I saw the folder labelled "University of Sussex - Cairngorms Field Trip - September 2018". It sat there like an accusation. I could neither throw it away nor bear to read it.

It was a physical reminder of something inexplicable, something that contradicted everything I understood about the natural world.

Then, in early January 2019, a letter arrived. The postmark was from northern England, and I recognised the handwriting at once.

Callum Reed.

My heart raced as I tore open the envelope and scanned the cramped lines. The letter was dated 28 December 2018, almost four months after the incident at Loch Avon.

Callum wrote that he had spent two weeks in isolation at Raigmore Hospital before being released. The doctors had given him a clean bill of health and claimed the infection had responded to aggressive antibiotic treatment, although they had never identified its cause.

For a time, he had believed them.

He returned south and tried to rebuild his life. The university arranged for him to complete the requirements of the ecology module through independent work. During the first few weeks, everything seemed normal.

Then the hunger began.

Callum described it as an overwhelming, gnawing emptiness that ordinary food could not satisfy. He ate more, changed his diet and tried vitamin supplements. Nothing helped.

The hunger grew stronger. With it came restlessness, heightened senses and disturbing dreams that dissolved the moment he woke.

My hands shook as I read what had happened next.

One night, while his girlfriend Lucy was visiting, Callum lost control. He attacked her. When he came back to himself, he was covered in her blood.

Lucy was taken to hospital with injuries blamed on a large stray dog that had supposedly entered through an unsecured rear door. Callum told the authorities that he had fought the animal off, but he knew the truth.

There had been no dog.

He had attacked Lucy himself.

The final pages were a desperate plea for help. Callum was terrified that he would hurt someone else. He had retreated to his family's remote stone cottage in an isolated North Pennines valley, where he hoped he could keep away from other people while he tried to understand what he was becoming.

"I can feel it changing me," he had written in a trembling hand.

"Whatever infected Rebecca and Owen is still inside me, Martin. It never left. The doctors didn't cure it. They only slowed it down.

"Please, if you know anything more about what happened to us, anything that might help me understand this hunger and this transformation, I need to know before I become something I can't come back from."

He had included his mobile number and the cottage address, begging me to contact him if I had any insight.

By late January 2019, the letter had been on my desk for about three weeks. I had read it dozens of times and called the number repeatedly, but every call went to voicemail. I had considered travelling to the North Pennines to find him, yet what could I possibly say?

I had no answers, only the same memories that poisoned my sleep. I could not explain what had infected Rebecca, Owen and Callum, or why Callum's transformation had taken months rather than minutes.

I sat at my desk in the cottage near Grantown-on-Spey, the letter open before me and snow falling softly beyond the window. The white Cairngorms filled the distance. Soon I was meant to leave for winter walking work in Dartmoor and the Brecon Beacons, but for the first time in my career I dreaded the wild places where I had always found solace.

Beside the letter lay the open file: contracts, medical forms, emergency contacts, the route plan and every other document that made the expedition appear, on paper, like any of the university field trips I had guided over the years.

But something had gone terribly wrong at Loch Avon. Something had transformed Rebecca, then Owen, and had remained dormant inside Callum before awakening as a hunger for human flesh.

In twenty-two years of guiding, I had never encountered anything like it.

Callum had asked me for help, and I still had no idea how to save him.