That all the passengers should suddenly, and so vehemently, be concerned with Theodore Mahler's welfare seemed, on the face of it, inexplicable. But only on the face of it. It did not require a great deal of thought or probing beneath the surface to discover that the true motivating factor was not selflessness – though there may have been some of that, too – but selfishness. Mahler represented not so much a sufferer as a most welcome diversion from their own thoughts and suspicions, from the tension, from the never-ending constraint that had laid its chilling hand over the entire company for the past twelve hours.

This constraint, apart from its awkwardness and sheer unpleasantness, had the further effect of splitting up the passengers into tiny groups. Communal speech had ceased entirely, except where necessity and the barest demands of common politeness made it inevitable.

Marie LeGarde and Margaret Ross, each of whom knew that the other was not under suspicion, kept very much to themselves and talked only between themselves. So, too, did Zagero and Solly Levin, and also – though this would have seemed ridiculously improbable only twenty-four hours ago – Mrs Dansby-Gregg and her maid, Helene. Improbable then, but inevitable now: whether guilty or not, both knew exactly where the other stood, and, of all the passengers, each could only fully trust the other. They could, of course, as could all the others, trust Marie LeGarde and Margaret Ross: but the fact that they knew that Marie LeGarde and Margaret Ross couldn't trust them was enough to prohibit any attempts to establish an easier relationship. As for Corazzini, the Rev. Small wood, the Senator and Mahler, they kept very much to themselves.

In the circumstances, then, it was inevitable that they should welcome the introduction of an absolutely innocuous subject of interest and conversation, something that would ease, however slightly, the coldness and discomfort of the social atmosphere, something that would divert their unwelcome and suspicious thoughts into some more tolerable channel. Theodore Mahler promised to be the best looked after patient I had ever had.

I had just got the oil stove going to my satisfaction when Zagero called to me from his seat by the rear canvas screen.

"There's somethin' funny goin' on outside, Doc. Come and have a look."

I had a look. Far off to the right – the north-west, that was – and high above the horizon a great diffuse formless volume of luminosity, spreading over almost a quarter of the dark dome of the sky, was beginning to pulse and fade, pulse and fade, strengthening, deepening, climbing with the passing of every moment. At first it was no more than a lightening in the sky, but already it was beginning to take form, and faint colours beginning to establish themselves in definite patterns.

"The Aurora, Mr Zagero," I said. "The Northern Lights. First time you've seen it?"

He nodded. "Yeah. Amazin' spectacle, ain't it?"

"This? This is nothing. It's just starting up. It's going to be a curtain – you get all sorts, rays, bands, coronas, arcs and what have you, but this is a curtain. Best of the lot."

"Get this sort of thing often, Doc?*

"Every day, for days on end, when the weather is like this – you know, cold and clear and still. Believe it or not, you can even get so used to it that you won't bother looking."

"I don't believe it. It's amazin'," he repeated, "just amazin'. Tired of it, you say -1 hope we see it every day." He grinned. "You don't have to look, Doc."

"For your own sake you'd better hope for something else," I said grimly.

"Meanin'?"

"Meaning that radio reception is hopeless when the aurora is on."

"Radio reception?" He crinkled his brows. "What we gotta lose, with the radio set in the cabin smashed and your friends in the trail party gettin' further away every minute? You couldn't raise either of them anyway."

"No, but we can raise our Uplavnik base when we get a bit nearer the coast," I said, and the next moment I could have bitten my tongue off. I had never even thought of the matter until then, but as soon as the words were out I realised that I should have kept this piece of knowledge to myself. The chances of Uplavnik listening in at the right time and on the right frequency were remote enough, but it was always a chance: we could have sent out a warning, summoned help long before the killers would have thought of making a break for it. But, now, if Zagero were one of the killers, he would make good and certain that the set would be smashed long before we got within radio range of the Uplavnik base.

I cursed myself for a blundering idiot, and stole a quick glance at Zagero. In the light streaming out from the gap in the curtain and in the fainter light of the aurora, his every feature was plain, but I could tell nothing from his expression. He was playing it casual, all right, but not too stupidly casual. The slow nod, the pursing of the lips, the thoughtful lifting of the eyebrows could not have been improved upon. Not even the best professional actor could have improved on it, and hard on the heels of that came the second thought that there were a couple of extraordinarily fine actors among us. But, then, if he hadn't reacted at all, or had reacted too violently, I would have been doubly suspicious. Or would I? If Zagero were one of the guilty men, wouldn't he have known that too much or too little reaction would have been the very thing to excite suspicion, and taken due precaution against registering either? I gave it up and turned away. But in my mind there was growing a vague but steadily strengthening suspicion against Johnny Zagero: and on the basis of the success and validity of my previous suspicions, I thought bitterly, that just about guaranteed Zagero's innocence.

I turned and touched Margaret Ross on the shoulder.

"I'd like to have a few words with you, Miss Ross, if you don't mind the cold outside."

She looked at me in surprise, hesitated for a moment, then nodded. I jumped down, reached up a hand to steady her, then helped her aboard the big sled as it passed by a few seconds later. For a short time we just sat there, side by side on a petrol drum, watching the aurora while I wondered how to begin. I stared almost unseeingly at the tremendous sweep of the developing aurora, the great folded, fluted curtain of yellow-green with red-tipped feet that seemed almost to brush the surface of the ice-cap, a translucent transparent drapery – for even at its brightest the stars still shone faintly through – that waved and shimmered and pulsed and glowed, a pastel poem in insubstantia-lity, like the ethereal backdrop to some unimaginably beautiful fairyland. Margaret Ross sat there gazing at it like one lost in a trance. But she might have been looking at it with the same uncaring eyes as myself, lost not in wonder but in the memory of the man we had left behind in the ice-cap. And when she turned at the sound of my voice, and I saw the glow of the aurora reflected in the sad depths of the wide brown eyes, I knew I was right.

"Well, Miss Ross, what do you think of the latest development?"

"Mr Mahler?" She'd slipped up her snow-mask – in her case just a gauze and cotton-wool pad with a central breathing aperture -and I had to lean forward to catch her soft voice. "What can one say about anything so-so dreadful. What chance does the poor man have, Dr Mason?"

"I've honestly no idea. There are far too many unpredictable factors involved.. . . Did you know that after I'd crossed you off I'd lined him up as number one on my list of suspects?"

"No!"

"But yes, I'm afraid. I fear I'm no sleuth, Miss Ross. I may be long on the empirical, trial and error method – and it at least has had the negative advantage of reducing the number of suspects by two – but I'm pretty short on the deductive." I told her what had happened between Mahler and myself during the brief stop we had made.