The snarl of the engine broke the silence so suddenly I jumped. The sound of a cold snowmobile being pulled to life. Once, twice, and then the steady roar of the running engine.

I hurried through the boot room and opened the main door. The cold hit me like a concrete wall and made my eyes water. Through the tears, I saw a red brake light disappearing off towards the Lucia glacier, a dark figure hunched over the controls. With his back to me, wrapped up in the snowmobile suit and helmet, he could have been my twin brother and I wouldn’t have recognised him.

I retreated inside before the cold killed me, telling myself it was probably nothing. One of the students who’d drawn the short straw, checking some remote set of frozen instruments. I’d ask at breakfast and—

At the far end of the corridor, near the mess, a door opened. Someone stepped out, checking the corridor both ways as if he was crossing a road. No time for me to hide.

He approached. I tensed, full fight-or-flight mode, but it was only Quam. Not sure why I say ‘only’ — being base commander didn’t put him above suspicion.

‘Early to be up,’ he said, in the sort of too friendly voice headmasters use just before they reach for the cane.

‘I needed the loo.’

He nodded, and didn’t ask why I hadn’t used the toilet opposite my room.

‘Head OK?’

‘Better, thanks.’

‘Good.’

He seemed nervous, shifting on his feet, drumming his fingers in mid-air. Like a man with something on his mind. Maybe a guilty secret.

Suddenly, I really didn’t want to be alone in that long, long corridor with him. I stared at the rows of doors, willing someone to come out. I tried to step past him, muttering about getting back to bed. Quam shifted his weight a little, not so much as to be obvious, but enough to block my way.

‘You’ve got a son, it says on your file.’

‘Luke. He’s eight.’

‘You must miss him.’

I nodded, and balled my fists. Maybe it was paranoia, but I couldn’t get it out of my head he was making a threat.

Maybe not. ‘I’ve got two daughters,’ he said. ‘I don’t see them often.’

‘It’s a long way,’ I sympathised.

‘We come out here, we fight every day. Do you ever wonder if it’s worth it?’

‘It’s science, I suppose.’

‘You haven’t been here long.’ His chin jerked up; he looked at me as if he’d only just noticed me. ‘Still, you know what we have in common?’

‘Insomnia?’

‘This island’s trying to kill both of us.’

There’s not much I could say to that. ‘I’d better get some sleep.’

He shook his head, as if he’d been thinking about something quite different. ‘Of course.’

A gust of wind shook the Platform. Quam pressed his hand against the wall, bracing himself. ‘Wind’s getting up. There’s a storm coming.’

‘You’re blocking my way,’ I said politely.

Greta came to the lab after breakfast. She was carrying one of the heavy red ECW coats in her arms.

‘Going out?’

She unfolded it and held it up so I could see. On the left breast, above the Zodiac badge, she’d embroidered my name. For a woman who spends her life welding snowmobiles and shovelling snow, she does a surprisingly dainty stitch.

‘You shouldn’t have.’

‘Now we own you.’

I put it on. I stroked my fingertip over the corrugations of the thread. I suppose she did it for everyone; probably, it was just one more job ticked off on a list. But I felt ridiculously grateful, almost weepy. As if I belonged.

‘I might not be around long enough to make use of it.’

She didn’t comment one way or the other.

Wearing that coat indoors, I’d already started to sweat. I stuck my hands in the pockets. Deep and padded, but I felt something hard at the bottom. I pulled it out. A teddy bear in a grubby I ♥ NY T-shirt, with a ring in its head and a key.

‘You found it. By the crevasse,’ she reminded me.

‘Did I find out what it opens?’

‘There aren’t any locks on Utgard.’

The conversation came back. Me: It must have fallen out of Martin’s pocket.

And Greta, pointing to the footprints that had chased Hagger to the brink. Or maybe his.

I put it on the bench to think about.

Eastman and Kennedy are back from Vitangelsk. After being in a coma for two days, it’s good to have a doctor around. Both of them dropped in to see me, either side of a rather contentious staff meeting.

Kennedy came first. I was already up and poking around the lab. He asked how I was, though I could as easily have asked him. He looked terrible. Raccoon rings round his eyes, hair askew (not that any of us looked like much), and his face that shade of grey T-shirts go when they’ve been through the wash too often.

‘Rough night?’ I asked as he shone a torch in my eyes. ‘The bear must have been quite a shock.’

He almost took my eye out with the torch. ‘The bear?’ he repeated, as if it was something I wasn’t supposed to know.

‘I heard you had a close encounter with a polar bear yesterday.’

‘That.’ He nodded, as if he’d only just remembered. I wouldn’t have thought it was the sort of thing you forgot so quickly. ‘Yes.’

He made me stand on one leg, touch my toes, count backwards from thirteen.

‘Keeping busy?’ he said, pointing to the map I’d left out on the table.

‘Trying to tidy up some of Hagger’s loose ends.’ He’d seen the cluster of X’s at Echo Bay; there was no point trying to hide it.

‘I thought Anderson’s last assistant might be able to fill me in. Kevin Maart.’

‘K-Mart.’ Kennedy chuckled. ‘South African, mad as pants. He used to wander around the Platform in his flip-flops. Hated the cold.’

‘I heard he left because of a wisdom tooth.’

Kennedy fiddled with a pen. ‘That’s right.’

‘Quam won’t give me his email address. I thought maybe you might have contact information. For emergencies, or whatever.’ I gave him a smile, to show I didn’t mean to impose. Kennedy didn’t return it.

‘I’ll see what I can do,’ he said. And left quickly.

I went back to the map and pencilled in a line that charted the current Ash had told me about. Where the current flowed on the west coast, between the Helbreensfjord and Echo Bay, the whole food chain was exploding, Ash said. But these little creatures, Gelidibacter incognita, were localised in Echo Bay.

I flipped through the notebook until I found a graph labelled ‘Propagation rates — Echo Bay’. It wasn’t hard to interpret: it swung up like a ski jump. Whatever was propagating, they were breeding like rabbits. I assumed it was the micro-organisms I’d seen under the microscope.

But the bugs weren’t in the upstream samples. That meant they weren’t floating down from anywhere: they were growing in Echo Bay.

Why?

Not my question: Hagger’s. Scrawled under the chart, heavily underscored.

I tried to think logically about what Hagger would have done. I stared at the graph. The time series across the bottom was measured in hours — terrifyingly fast.

I flipped over the page. On the back were two more graphs. One was a copy of the previous page, the vertiginous swoop up as the bugs replicated like crazy. But underneath, a second graph on the same scale painted a different picture: just a flat line across the page tapering towards zero.

Sometimes they thrived, sometimes they died.

The two charts were labelled X-positive and X-negative.

What is X?

Again, Hagger’s question. I didn’t know either. It must have something to do with the red and green dots on the samples.

I found two clean beakers and set them up on the bench. One, I filled with 100ml of water, a red sample from the Helbreensfjord. The other, I filled with the same amount of water from a green Zodiac sample.

After a moment’s thought, I found a third beaker and filled it with tap water, as a control. I sterilised a pipette and took three 10ml extracts from one of the Echo Bay samples. I squirted them out, one for each beaker, and stirred them up.