We tramped over the crisp snow to the edge of town. The noise got louder. On my left, the pylons marched away across the ridge. Down the slope, the sun gleamed off a lone snowmobile racing up the valley from the south.

We crouched behind the snow that had drifted against the barracks. Eastman handed me his binoculars and pointed. ‘Is that the guy?’

Through the binoculars, the snowmobile jumped into focus. I could even read the manufacturer’s name: Polaris, the same as we used at Zodiac. A hunched figure in a red suit straddled it, though the glare on his visor hid his face.

‘He looks too small,’ I said. But fear grows in memory. Perhaps I’d misremembered him.

Eastman took a turn with the binoculars. ‘He’s hammering that thing.’

We ducked lower. I expected the snowmobile to turn towards Vitangelsk — there was nowhere else to go — but instead it carried straight on, to the end of the valley and up towards the ridge. The engine whined as it fought the steep slope.

Eastman lay flat on the snow bank. I lay beside him, and watched the snowmobile crawl on up. It swerved this way and that, either to avoid obstacles I couldn’t see, or perhaps because the driver was looking for something.

Eastman gave me the binoculars again. I needed a moment to pick out the snowmobile; when I did, it had stopped, just below the line of the cableway.

The driver got off, walking in that stiff, bandy-legged way you do when you’ve been riding a snowmobile for hours. He must have come from Zodiac. The snowmobile suit, the machine, the rifle on his back — they were all standard-issue equipment.

Could it be the man from yesterday? Why would he have gone back, got a snowmobile, changed his clothes? But who else would come here at this time of morning?

He lifted off his helmet and put it on the seat. He had his back to me, and his balaclava hid his hair. I waited for him to turn around.

‘Can you see who it is?’ Eastman reached for the glasses, but I kept hold of them. I had to know.

And then the man disappeared. One moment he was a bright red blot against the snowfield. The next, he’d vanished.

Even with the naked eye, Eastman could see he’d gone.

‘Did he see us?’

I shrugged. ‘He must have gone behind a rock. Or down a gully.’ I scanned the hillside with the binoculars. All I saw was snow, and the abandoned snowmobile.

‘If we can’t see him …’ Eastman climbed over the snow bank and started galloping across the hillside. After a moment, and checking I had the flare gun in my pocket, I followed.

At that altitude, the wind had scoured most of the snow off the rocks, but the slope made it heavy going. After a hundred yards, I was puffing; after two hundred, the cold air rasping my lungs made me want to vomit. In the clear air, I could see the snowmobile pin sharp, but it never seemed to get any closer.

Eastman got there first. I caught him up a minute or so later. He pointed to the Zodiac number stencilled on the snowmobile’s cowling.

‘Definitely one of ours.’

Footprints led away towards a rocky overhang. Orange-brown marks discoloured the snow like a rash. It could have been lichen — there are types that grow in snow and spread like stains — but these weren’t like that. They looked like blood.

Under the overhang, a dark hole opened in the mountainside. Sunk in a hollow, angled away from Vitangelsk, invisible until you were virtually in it. Snow had collected by the entrance, and I could see footsteps leading in, as well as the corrugations of an old snowmobile track. And more of the stains, thicker and bloodier than before.

Eastman aimed the rifle at the cave. I cocked the flare gun.

‘Who’s there?’ Eastman shouted. I hoped he was as confident as he sounded.

Silence. Then a shuffling noise from inside the cave, the clatter of stones. I had a vision of some primal monster woken from sleep, Yeats’s rough beast slouching towards Bethlehem. Or maybe nothing so fanciful. Bears live in caves, after all.

A figure appeared in the blue light around the entrance. He looked shorter than the man who’d chased me up the pylon. I couldn’t see a gun, but I wasn’t taking any chances. I gripped the flare pistol tighter.

He pulled off his balaclava and rubbed his eyes. He stepped into daylight, arms half-raised in a bemused way, as if he couldn’t believe there was really a gun pointing at him. With his red suit, black boots, white beard and pot belly, he looked like nothing so much as Father Christmas.

The pistol shook in my hand.

‘Ash?’

Ash sat on a rock and rubbed snow out of his beard. If it surprised him to have two colleagues pointing guns at him at four in the morning, here on the upper edge of nowhere, he kept it to himself. Perhaps he expected it.

‘How did you guess?’ he said. No pretending.

‘Jensen told us he dropped you off here. We worked the rest out ourselves.’

‘I thought he might. I could see he wasn’t happy, not after what happened to Hagger.’

A pause.

‘You killed him,’ said Eastman.

Ash closed his eyes and nodded silently.

‘Why?’ I wanted to know.

‘I had no choice. He came at me, I had to protect myself.’

‘Why?’ I repeated. Eastman cut me off.

‘What about DAR-X? They were there too?’

‘They’d been around. I saw their Sno-Cat. I don’t think they saw me.’

‘And then you went back and pretended nothing had happened.’

He shrugged. ‘What else could I do? It would have been the end of my career if I’d confessed I shot him.’

That gave me a jolt — like a spelling mistake that jars you out of a book.

‘What are we talking about?’ I said.

Ash looked puzzled ‘What are we talking about?’

‘Martin Hagger,’ said Eastman. ‘And why you killed him.’

Ash blinked. He looked slowly between me and Eastman, started to say something, then shook his head. Strange to say, he was smiling.

‘You think I killed Hagger?’

‘You just admitted it,’ said Eastman.

Ash stood and turned towards the cave. Eastman’s rifle twitched, but it didn’t seem to bother him any more.

‘I’ll show you.’

Eastman and I followed Ash in with our head torches. The cave was just high enough to stand in, if you stooped. Perhaps it had been an attempt at a mineshaft; if so, they hadn’t got very far. A few metres in I could see a corrugated-iron wall blocking off the passage, with a heap of snow blown against its base.

Except the wall wasn’t corrugated iron. As my torch caught it, I saw colours, writing. Pictures of broccoli and tomatoes, spaghetti letters and smiling beans.

It was cans. Tin cans, all stacked up as you might find them at Aldi. Soups, vegetables, baked beans, spaghetti hoops — the whole fifty-seven varieties. So many, they walled off the back of the cave.

‘You been stealing from the kitchen?’ Eastman asked.

Ash looked as if he was about to cry. He shook his head and pointed to the floor. Then I understood.

The wall wasn’t corrugated iron — and the wind-blown snow at its base wasn’t snow.

Too soft; more yellow than white. As I shone the torch beam down, I made out two legs, the crease of a floppy tail. Further forward, I could see an outflung paw and a black nose resting on it. Much smaller than the bear that had chased me the day before. Just a cub.

Eastman got it a second before me. ‘Jesus Christ, Ash. You shot a baby polar bear?’

‘When did it happen?’ I asked.

‘The day Hagger died.’

‘Why didn’t you tell us?’

‘I’m a zoologist. What do you think would happen to my career if it came out I’d shot a polar bear cub. I might as well take up whaling.’

‘But you said it was in self-defence.’

‘As if they’d care about the details.’

Eastman wiped his face. Out of the sun, the sweat he’d built up running had started to freeze. He was shivering.