"But – but now they know you've seen too much and know too much." I wished she wouldn't look at me when she was talking, these eyes would have made even the Rev. Smallwood forget himself in the middle of his most thundering denunciations – not that I could imagine Mr Smallwood going in for thundering denunciations very much.

"A disquieting thought," I admitted, "and one that has occurred to me several times during the past half-hour. About five hundred times, I would say."

"Oh, stop it! You're probably as scared as I am." She shivered. "Let's get out of here, please. It's – it's ghastly, it's horrible. What – what was that?" Her voice finished on a sharp high note.

"What was what?" I tried to speak calmly, but that didn't stop me from glancing around nervously. Maybe she was right, maybe I was as scared as she was.

"A noise outside." Her voice was a whisper and her fingers were digging deep into the fur of my parka. "Like someone tapping the wing or the fuselage."

"Nonsense." My voice was rough, but I was on razor-edge. "You're beginning to – "

I stopped in mid-sentence. This time I could have sworn I had heard something, and it was plain that Margaret Ross had too. She twisted her head over her shoulder, looking in the direction of the noise, then slowly turned back to me, her face tense, her eyes wide and staring.

I pushed her hands away, reached for gun and torch, jumped up and started running. In the control cabin I checked abruptly -God, what a fool I'd been to leave that searchlight burning and lined up on the windscreens, blinding me with its glare, making me a perfect target for anyone crouching outside with a gun in hand – but the hesitation was momentary only. It was then or never – I could be trapped in there all night, or until the searchlight battery died. I dived head first through the windscreen, caught a pillar at the very last moment and was lying flat on the ground below in less time than I would have believed possible.

I waited five seconds, just listening, but all I could hear was the moan of the wind, the hiss of the ice spicules rustling along over the frozen snow – I'd never before heard that hissing so plainly, but then I'd never before lain with my uncovered ear on the ice-cap itself – and the thudding of my heart. And then I was on my feet, the probing torch cutting a bright swathe in the darkness before me as I ran round the plane, slipping and stumbling in my haste. Twice I made the circuit, the second time in the opposite direction, but there was no one there at all.

I stopped before the control cabin and called softly to Margaret Ross. She appeared at the window, and I said: "It's all right, there's no one here. We've both been imagining things. Come on down." I reached up my hands, caught her and lowered her to the ground.

"Why did you leave me up there, why did you leave me up there?" The words came rushing out, tumbling frantically one over the other, the anger drowned in the terror. "It was – it was horrible! The dead man. . . . Why did you leave me?"

"I'm sorry." There was a time and a place for comment on feminine injustice, unreasonableness and downright illogicality, but this wasn't it. In the way of grief and heartbreak, shock and ill-treatment, she had already had far more than she could stand. "I'm sorry," I repeated. "I shouldn't have done it. I just didn't stop to think."

She was trembling violently, so I put my arms round her and held her tightly until she had calmed down, took the searchlight and battery in one hand and her hand in my other and we walked back to the cabin together.

Jackstraw and the others had just completed the assembly of the tractor body when we arrived back at the cabin, and some of the men were already going below. I didn't bother to check the tractor: when Jackstraw made anything, he made a perfect job of it.

I knew he must have missed me in the past hour, but I knew, too, that he wasn't the man to question me while the others were around. I waited till the last of these had gone below, then took him by the arm and walked out into the darkness, far enough to talk in complete privacy, but not so far as to lose sight of the yellow glow from our skylights – twice lost in the one night was twice too many.

He heard me out in silence, and at the end he said: "What are we going to do, Dr Mason?"

"Depends. Spoken to Joss recently?"

"Fifteen minutes ago. In the tunnel."

"How about the radio?"

"I'm afraid not, Dr Mason. He's missing some condensers and spare valves. He's looked for them, everywhere – says they've been stolen."

"Maybe they'll turn up?" I didn't believe it myself.

"Two of the valves already have. Crushed little bits of glass lying in the bottom of the snow tunnel."

"Our little friends think of everything.1'! swore softly. "That settles it, Jackstraw. We can't wait any longer, we'll leave as soon as possible. But first a night's sleep – that we must have."

"Uplavnik?" That was our expedition base, near the mouth of the Stromsund glacier. "Do you think we will ever get there?"

He wasn't thinking, just as I wasn't, about the rigours and dangers of arctic winter travel, daunting enough though these were when they had to be faced with a superannuated tractor like the Citroen, but of the company we would be keeping en route. If any fact was ever so glaringly obvious that it didn't need mention, it was that the killers, whoever they were, could only escape justice, or, at least, the mass arrest and interrogation of all the passengers, by ensuring that they were the only ones to emerge alive from the ice-cap.

"I wouldn't like to bet on it," I said dryly. "But I'd bet even less on our chances if we stay here. Death by starvation is kind of final."

"Yes, indeed." He paused for a moment, then switched to a fresh line of thought. "You say they tried to kill you tonight. Is that not surprising? I would have thought that you and I would have been very safe, for a few days at least."

I knew what he meant. Apart from Jackstraw and myself, there probably wasn't a handful of people in all Greenland who could start that damned Citroen, far less drive it, only Jackstraw could handle the dogs, and it was long odds indeed against any of the passengers knowing anything at all about astral or magnetic compass navigation – the latter very tricky indeed in these high latitudes. These special skills should have been guarantee enough of our immediate survival.

"True enough," I agreed. "But I suspect they haven't given any thought to these things simply because they haven't realised the importance of them. We'll make it our business to point out that importance very plainly. Then we're both insured. Meantime, we'll have one last effort to clear this business up before we get started. It's not going to make us very popular, but we can't help that." I explained what I had in mind, and he nodded thoughtful agreement.

After he had gone below, I waited a couple of minutes and then followed him. All nine of the passengers were sitting in the cabin now – eight, rather, watching Marie LeGarde presiding over a soup pan – and I took a long, long look at all of them. It was the first time I had ever examined a group of my fellow-men with the object of trying to decide which among them were murderers, and found it a strange and unsettling experience.

In the first place, every one of them looked to me like a potential or actual murderer – or murderess – but even with that thought came the realisation that this was purely because I associated murder with abnormality, and in these wildly unlikely surroundings, clad in the layered bulkiness of these wildly unlikely clothes, every one of them seemed far removed from normality. But on a second and closer look, when one ignored the irrelevancies of surroundings and clothes, there remained only a group of shivering, feet-stamping, miserable and very ordinary people indeed.