Or were they so ordinary? Zagero, for instance, was he ordinary? He had the build, the strength and, no doubt, also the speed and temperament for a top-ranking heavyweight, but he was the most unlikely looking boxer I had ever seen. It wasn't just that he was obviously a well-educated and cultured man – there had been such boxers before: it was chiefly because his face was absolutely unmarked, without even that almost invariable thickening of skin above the eyes. Moreover, I had never heard of him, although that, admittedly, didn't go for much: as a doctor, I took a poor view of homo sapiens wreaking gratuitous physical and mental injury on homo sapiens, and took little interest in the sport.

Or take his manager, Solly Levin, or, for that matter, the Rev. Joseph Small wood. Solly wasn't a New York boxing manager, he was a caricature of all I had ever heard or read about these Runyonesque characters, and he was just too good to be true: so, also, was the Rev. Small wood, who was so exactly the meek, mild, slightly nervous, slightly anaemic man of God that preachers are so frequently represented to be – and almost invariably never are -that his movements, reactions, comments and opinions were predictable to the nth degree. But, against that, I had to set the fact that the killers were clever calculating men who would have carefully avoided assuming the guise of any character so patently cut from cardboard: on the other hand, they might have been astute enough to do just that.

There was a question mark, too, about Corazzini. America specialised in producing shrewd, intelligent, tough business leaders and executives, and Corazzini was undoubtedly one such. But the toughness of the average business man was purely mental: Corazzini had physical toughness as well, a ruthlessness I felt he wouldn't hesitate to apply to matters lying far outside the immediate sphere of business. And then I realised, wryly, that I was prepared to suspect Corazzini for reasons diametrically opposed to those for which I was prepared to suspect Levin and the Rev. Smallwood: Corazzini didn't fit into any pattern, any prefabricated mental image of the American business man.

Of the two remaining men, Theodore Mahler, the little Jew, and Senator Brewster, I would have taken the former any time as the more likely suspect. But when I asked myself why, I could adduce no more damaging reasons than that he was thin, dark, rather embittered looking and had told us absolutely nothing about himself: and if that weren't prejudice on my part, I couldn't guess what was. As for Senator Brewster, he was surely above suspicion: and then the startling thought struck me that if one wished to be above suspicion surely there were no better means of achieving that than by assuming the identity of someone who was above suspicion. How did I know he was Senator Brewster? A couple of forged papers, a white moustache and white hair on top of a naturally florid complexion and anyone could have been Senator Brewster. True, it would be an impersonation impossible to sustain indefinitely: but the whole point was that any such impersonation didn't have to be sustained indefinitely.

I was getting nowhere and I knew it: I was more confused, more uncertain, and infinitely more suspicious than ever. I was even suspicious of the women. The young German girl, Helene -Munich was her home town, near enough Central Europe and the skulduggery that went on in the neighbourhood of the iron curtain for anything to be possible: but on the other hand the idea of a seventeen-year-old master criminal – we certainly weren't dealing with apprentices – was ridiculously far-fetched, and the fact that she had fractured her collar-bone, almost sure proof that the crash had been unexpected, was a strong point in her favour. Mrs Dansby-Gregg? She belonged to a world I knew little about, except for what slight information I had gleaned from my psychiatric brethren, who found rich fishing in the troubled waters of what passed for the younger London society: but instability and neuroses – not to mention the more than occasional financial embarrassment – were not criminal in themselves, and, in particular, that world lacked what people like Zagero and Corazzini had in full measure – the physical and mental toughness required for a job like this. But particularising from the general could be every bit as dangerous and misleading as generalising from the particular: of Mrs Dansby-Gregg, as a person, I knew nothing.

That left only Marie LeGarde. She was the touchstone, the one rock I could cling to in this sea of uncertainty, and if I were wrong about her so too had been a million others. There are some things that cannot be because they are unthinkable, and this was one of them. It was as simple as that. Marie LeGarde was above suspicion.

I became gradually aware of the muted clack of the anemometer cups turning sluggishly in the dying wind above and that the hiss of the Colman lamp had become abnormally loud: a total silence had fallen over the cabin and everyone was staring at me with mingled puzzlement and curiosity. So much for my impassive features, my casual negligent ease: so clearly had I betrayed the fact that something was far wrong that not one of the nine had missed it. But to be the centre of attraction at the moment suited me well enough: Jackstraw had just made his, entry unobserved, a Winchester repeater cradled under his arm, his finger ready through the trigger guard.

"Sorry," I apologised. "Rude to stare, I know. However, now it's your turn." I nodded in Jackstraw's direction. "Every expedition carries a gun or two – for coast use against prowling bears and wolves and to get seal meat for the dogs. I never thought that it would come in so handy right in the middle of the ice-cap – and against far more dangerous game than we ever find on the coast. Mr Nielsen is a remarkably accurate shot. Don't try anything -just clasp your hands above your heads. All of you."

As if controlled by a master switch, all the eyes had now swivelled back to me. I'd had time to spare to pull out the automatic – a 9 mm butt-loading Beretta – that I'd taken off Colonel Harrison: and this time I didn't forget to slide off the safety-catch. The click was abnormally loud in the frozen silence of the room. But the silence didn't last long.

"What damnable outrage is this?" Senator Brewster shouted out the words, his face purpling in rage. He leapt to his feet, started to move forwards towards me then stopped as if he had run into a brick wall. The crash of Jackstraw's Winchester was a deafening, eardrum shattering thunderclap of sound in that confined space: and when the last reverberations of the rifle-shot had faded and the smoke cleared away, Senator Brewster was staring down whitely at the splintered hole in the floor boards, almost literally beneath his feet: Jackstraw must have miscalculated the Senator's rate of movement, for the bullet had sliced through the edge of the sole of Brewster's boot. However it was, the effect couldn't have been bettered: the Senator reached back blindly for the support of the bunk behind him and lowered himself shakily to his seat, so terrified that he even forgot to clasp his hands above his head. But I didn't care about that: there would be no more trouble from the Senator.

"OK, so you mean business. Now we're convinced." It was Zagero who drawled out the words, but his hands were tightly enough clasped above his head. "We know you wouldn't do this for nothin', Doc. What gives?"

"This gives," I said tightly. "Two of you people are murderers -or a murderer and murderess. Both have guns. I want those guns."

"Succinctly put, dear boy," Marie LeGarde said slowly. "Very concise. Have you gone crazy?"

"Unclasp your hands, Miss LeGarde, you're not included in this little lot. No, I'm not crazy. I'm as sane as you are, and if you want evidence of my sanity you'll find it out on the plane there -or buried out on the ice-cap: the captain of the plane with a bullet through his spine, the passenger in the rear with a bullet through his heart and the second officer smothered to death. Yes, smothered. Not cerebral haemorrhage, as I said: he was murdered in his sleep. Believe me, Miss LeGarde? Or would it take a personal tour of the plane to convince you?"