We were through without further incident and at the head of that long almost imperceptible slope that fell away for thousands of feet towards the ice-bare rocks of the Greenland coast, before the last of the noon twilight had faded from the sky. I halted the tractor, spoke briefly to Jackstraw, told Margaret Ross to start thawing out some corned beef for our belated mid-day meal, and had just seen Mahler, now semi-conscious, and Marie LeGarde once again safely ensconced in the tractor cabin when Margaret Ross came up to me, her brown eyes troubled.

"The tins, Dr Mason – the corned beef. I can't find them."

"What's that? The bully? They can't be far away, Margaret." It was the first time I'd called her that, but my thoughts had been fixed exclusively on something else, and it wasn't until I saw the slight smile touching her lips – if she was displeased she was hiding it quite well – that I realised what I had said. I didn't care, it was worth it, it was the first time I had ever seen her smile, and it transformed her rather plain face – but I told my heart that there was a time and a place for somersaults, and this wasn't it. "Come on, let's have a look."

We looked, and we found nothing. The tins were gone all right. This was the excuse, the opportunity I had been waiting for. Jackstraw was by my side, looking at me quizzically as we bent over the sled, and I nodded. "Behind him," I murmured.

I moved back to where the others were grouped round the rear of the tractor cabin and took up a position where I could watch them all – but especially Zagero and Levin.

"Well," I said, "you heard. Our last tins of beef have gone. They didn't just vanish. Somebody stole them. That somebody had better tell me, for I'm going to find out anyway."

There was an utter silence that was broken only occasionally by the stirring of the dogs on the tethering cable. No one said anything, no one as much as looked at his neighbour. The silence stretched on and on, then, as one man, they all swung round startled at the heavy metallic click from behind them. Jackstraw had just cocked the bolt of his rifle, and I could see the slow stiffening of Zagero's back and arms as he realised that the barrel was lined up on his own head.

"It's no coincidence, Zagero," I said grimly. I had my own automatic in my hand by the time he turned round. "That rifle's pointed just where it's meant to. Bring your bag here."

He stared at me, then called me an unprintable name.

"Bring it here," I repeated. I pointed the Beretta at his head. "Believe me, Zagero, I'd as soon kill you as let you live."

He believed me. He brought the bag, flung it at my feet.

"Open it," I said curtly.

"It's locked."

"Unlock it."

He looked at me without expression, then searched through his pockets. At last he stopped and said, "I can't find the keys."

"I'd expected nothing else. Jackstraw – " I changed my mind, one gun was not enough to cover a killer like Zagero. I glanced round the company, made my choice. "Mr Small wood, perhaps you – "

"No, thank you," Mr Smallwood said hastily. He was still holding a handkerchief to his puffed mouth. He smiled wryly. "I've never realised so clearly before now how essentially a man of peace I am, Dr Mason. Perhaps Mr Corazzini – "

I glanced at Corazzini, and he shrugged indifferently. I understood his lack of eagerness. He must have known that I'd had him high up among my list of suspects until very recently indeed and a certain delicacy of sentiment might well prevent him from being too forthcoming too soon. But this was no time for delicacy. I nodded, and he made for Zagero.

He missed nothing, but he found nothing. After two minutes he stepped back, looked at me and then, thoughtfully, at Solly Levin. Again I nodded, and again he began to search. In ten seconds he brought out a bunch of keys, and held them up.

"It's a frame-up," Levin yelped. "It's a plant! Corazzini tnusta palmed 'em and put 'em there. I never had no keys – "

"Shut up!" I ordered contemptuously. "Yours, Zagero?"

He nodded tightly, said nothing.

"OK, Corazzini," I said. "Let's see what we can find."

The second key opened the soft leather case. Corazzini dug under the clothes on top and brought out the three corned beef tins.

"Thank you," I said. "Our friend's iron rations for his take-off. Miss Ross, our lunch. . . . Tell me, Zagero, can you think of any reason why I shouldn't kill you now?"

"You've made nothin' but mistakes ever since I met you," Zagero said slowly, "but, brother, this is the biggest you ever made. Do you think I would be such a damn fool as to incriminate myself that way? Do you think I would be so everlastingly obvious – "

"I think that's exactly the way you expected me to think," I said wearily. "But I'm learning, I'm learning. One more job, Corazzini, if you would. Tie their feet."

"What are you going to do?" Zagero asked tightly.

"Don't worry. The executioner will collect his fee. From now on you and Levin ride, with your feet tied, in the front of the tractor sled – and with a gun on you all the time. . . . What is it, Miss LeGarde?"

"Are you sure, Peter?" It was the first time she had spoken for hours, and I could see that even that tiny effort tired her. "He doesn't look like a murderer." The tone of her voice accurately reflected the expressions of consternation and shocked disbelief on half a dozen faces: Zagero had spared no effort to make himself popular with everyone.

"Does anybody here?" I demanded. "The best murderers never do." I then explained to her – and the others – all I knew and had suspected about everything. It shook them, especially the facts of the spiking of the petrol and of Hillcrest having been, at one time, only a few hours behind us: and by the time I was finished I could see that there was as little doubt in their minds about Zagero's guilt as there was in mine.

Two hours later, well down the slope from the Vindeby Nunataks, I stopped and set up the radio gear. I reckoned that we were now less than a hundred miles from the coast, and for half an hour tried to raise our base at Uplavnik. We had no success, but I had hardly expected any: the radio shack at the base was manned only by one operator, he couldn't be expected to be on watch all the time, and obviously his call-up bell wasn't set for the frequency I was using.

At four o'clock exactly I got through to Hillcrest. This time I hadn't bothered to move the radio out of hearing range -1 was actually leaning against the tractor cabin as I spoke – and every word said, both by Hillcrest and myself, could be clearly heard. But it didn't matter any more.

The first thing I did, of course, was to tell him that we had got our men. Even as I spoke, my own voice sounded curiously flat and lifeless. I should, I suppose, have been exultant and happy, but the truth was that I had suffered too much, both physically and psychologically, in the past few days, exhaustion lay over me like a smothering blanket, the reaction from the strain of those days was beginning to set in, the awareness was clearly with me that we weren't out of the wood by a long way yet, the lives of Marie LeGarde and Mahler were now the uppermost thoughts in my mind, and, to be perfectly honest, I also felt curiously deflated because I had developed a considerable liking for Zagero and the revelation of his true character had been more of a shock to me than I would have been prepared to admit to anyone.

Hillcrest's reactions, I must admit, were all that could have been wished for, but when I asked him about his progress the enthusiasm vanished from his voice. They were still bogged down, it seemed, and progress had been negligible. There was no word yet of passenger lists or of what the plane had carried that had been so important. The Triton, the aircraft-carrier, had insulin aboard and would fly it up to Uplavnik. A landing barge was moving into Uplavnik through an ice lead and was expected to arrive tomorrow and unload the tractor it was carrying, which would move straight out to meet us. Two ski-planes and two search bombers had been looking for us, but failed to locate us -we'd probably been traversing the Vindeby Nunataks at the time . . . His voice went on and on, but I hadn't heard anything he'd said in the past minute or so. I had just remembered something I should have remembered a long time ago.