"The answers to your questions are these. Firstly, the passenger list from BOAC in America is not yet through, but it is known from newspaper reports that the following three people were aboard: Marie LeGarde, the musical comedy star, Senator Hoffman Brewster of the United States and a Mrs Phyllis Dansby-Gregg, who appears to be a very prominent London socialite."

I wasn't greatly excited over this item of news. Marie LeGarde had never been a suspect. Mrs Dansby-Gregg – and, by implication, Helene Fleming – had never had more than a faint question mark against their names, and I had already come to the conclusion that it was long odds against the man who was, or purported to be, Senator Brewster being one of the killers.

"The second thing is this. The Admiralty cannot or will not say why the plane has been forced down, but I gather there must have been a most vital reason. Uplavnik suggests, on what basis I cannot say, perhaps it is officially inspired, that some person aboard the plane must have been in possession of something of the utmost importance, so important that complete secrecy was vital. Don't ask me what it was. A microfilm, a formula, something, perhaps, only committed to memory – it sounds fanciful, but that's all we can guess at. It does seem likely that Colonel Harrison was in possession of it."

I looked at Jackstraw, and he at me. The man who had so recently knocked me out had been desperate all right. I knew then what I had subconsciously known all along, that I was dealing blindfolded against a man – or men – far cleverer than myself. They knew that Joss couldn't possibly have hoped to repair the RCA. They knew, therefore, that I must have been talking direct to Hillcrest. They knew, because I had told them, that the eight-watt radio we had with us had a range of not more than 150 miles under normal conditions, so that the chances were high that Hillcrest was actually speaking from the IGY cabin – or a point even nearer. I had also told them that Hillcrest and his four companions wouldn't be returning from their field trip for another two or three weeks, so that this premature return could only be accounted for by some unforeseen and extraordinary event. It wasn't hard to guess what that event must have been. That I should ask Hillcrest to find out the reason for the crash followed inevitably, but what was not inevitable, what pointed most clearly of all to the shrewdness of the killers, was their guess that whoever knew the reason for the crash would be most reluctant to go into specific detail: and they had robbed me of the only clue that might have helped me discover what that detail was and so also, I felt sure, the identity of the killers. But the time was far past now for crying over spilt milk.

I pressed the switch to Transmit'.

"Thank you. But please radio Uplavnik again, emphasise desperate urgency of finding out crash reasons. . . . How far behind do you estimate you are now? We have made only twenty miles since noon. Cold extreme, bad radiator trouble. Over."

"We have made only eight miles since noon. It seems – "

I threw the switch over.

"Eight miles?" I demanded harshly. "Did I hear you say eight miles?"

"You heard." Hillcrest's voice was savage. "Remember the missing sugar? Well, it's turned up. Your fine friends dumped the whole bloody lot into the petrol. We're completely immobilised."

CHAPTER NINE – Wednesday 8 P.M. – Thursday 4 P.M.

We were on our way again just after nine o'clock that night. It had been my original intention, by dreaming up a variety of excuses and even, if necessary, by sabotaging the engine, to stay there for several hours or at least what I reckoned to be the longest possible time before the killers became restive, suspected that I was deliberately stalling, and took over. Or tried to take over. For it had been my further intention that, after an hour or two, Jackstraw should produce his rifle – it was strapped to his shoulders night and day – and I my automatic, and hold them all at the point of the gun until Hillcrest came up. If all had gone well, he should have been with us by midnight. Our troubles would have been over.

But it had not gone well, our troubles were as bad as ever, the Sno-Cat was bogged down and with Mahler now seriously ill and Marie LeGarde frighteningly weak and exhausted, I couldn't remain any longer. Had I been made of tougher stuff, or even had I not been a doctor, I might have brought myself to recognise that both Marie LeGarde and Theodore Mahler were expendable pawns in a game where the stakes, I was now certain, were far greater than just the lives of one or two people. I might have held everybody – or the major suspects, at least – at gunpoint until such time, twenty-four hours if need be, as Hillcrest did come up. But I could not bring myself to regard our sick passengers as expendable pawns. A weakness, no doubt, but one that I was almost proud to share with Jackstraw, who felt exactly as I did.

That Hillcrest would come up eventually I felt pretty sure. The dumping of the sugar in the petrol – I bit my lips in chagrin whenever I remembered that it had been I who had told them all that Hillcrest was running short of fuel – had been a brilliant move, but nothing more, now, than I had come to expect of men who thought of everything, made every possible provision against future eventualities. Still, even though furiously angry at the delay, Hillcrest had thought he could cope with the situation. The big cabin of the Sno-Cat was equipped with a regular workshop with tools fit to deal with just about every mechanical breakdown, and already his driver-mechanic-1 didn't envy him his murderous task even though he was reportedly working behind heated canvas aprons – had stripped down the engine and was cleaning pistons, cylinder walls and valves of the unburnt carbon deposits that had finally ground the big tractor to a halt. A couple of others had rigged up a makeshift distillation unit – a petrol drum, almost full, with a thin metal tube packed in ice leading from its top to an empty drum. Petrol, Hillcrest had explained, had a lower boiling point than sugar, and when the drum was heated the evaporating gas, which would cool in the ice-packed tube, should emerge as pure petrol.

Such, at least, was the theory, although Hillcrest didn't seem absolutely sure of himself. He had asked if we had any suggestion, whether we could help him in any way at all, but I had said we couldn't. I was tragically, unforgivably wrong. I could have helped, for I knew something that no one else did, but, at the moment, I completely forgot it. And because I forgot, nothing could now avert the tragedy that was to come, or save the lives of those who were about to die.

My thoughts were black and bitter as the tractor roared and lurched and clattered its way south-west by west under the deepening darkness of a sky that was slowly beginning to fill with cloud. A dark depression filled me, and a cold rage, and there was room in my mind for both. I had a strange fey sense of impending disaster, and though I was doctor enough to know that it was almost certainly a psychologically induced reaction to the cold, exhaustion, sleeplessness and hunger – and a physical reaction to the blow on the head – nevertheless I could not shake it off: and I was angry because I was helpless.

I was helpless to do anything to protect any of the innocent people with me, the people who had entrusted themselves to my care, the sick Mahler and Marie LeGarde, the quiet young German girl, the grave-faced Margaret Ross – above all, I had to admit to myself, Margaret Ross: I was helpless because I knew the murderers might strike at any time, for all I knew they might believe that Hillcrest had already told me all I needed to know ana that I was just waiting my chance to catch them completely off guard; on the other hand they, too, were almost certainly just biding their time, not knowing how much I knew, but just taking a calculated gamble, letting things ride as long as the tractor kept moving, kept heading in the right direction, but prepared to strike once and for all when the time came: and, above all, I was helpless because I still had no definite idea as to who the killers were.