"Don't move, Dr Mason." The voice, flat, controlled but alive with a vibrant power that I had never heard before, was almost unrecognisable as the Rev. Joseph Smallwood's. "Nobody is to move. Nielsen, drop that rifle – now! Just one suspicious move and Dr Mason gets his head blown off."

I stood stock-still. The man behind that voice meant every word he said. I didn't need any convincing of that. The cold certainty in his voice only reinforced the knowledge I already had that the sanctity of human life was a factor which could never enter into this man's considerations.

"All right, Corazzini?" Smallwood was speaking again, his voice empty of all concern for and interest in his accomplice: his only anxiety, if one could by any stretch of imagination call it that, lay in his desire for Corazzini's effectively continued co-operation.

"All right," Corazzini said softly. He was standing now and that both mind and reactions were back to normal was evident from the dexterity with which he caught the gun Smallwood threw back to him. "Never thought any man could move so fast with his feet tied. But he won't catch me again. Everybody out, eh?"

"Everybody out," Smallwood nodded. No question, he was the leader of the two, ridiculously improbable though that would have seemed only two minutes ago: but it didn't seem improbable any longer, it seemed inevitable.

"Jump down! All of you," Corazzini ordered. Gun in one hand, he held back a flap of canvas screen with the other. "Hurry it up."

"Mahler can't jump down," I protested. "He can't move – he's in coma. He – "

"Shut up!" Corazzini interrupted. "All right, Zagero, inside and get him out."

"You can't move him!" I shouted furiously. "You'll kill him if – " My last word was choked off in a grunt of pain as Smallwood's gun barrel caught me viciously across the side of the head. I fell to my hands and knees in the snow and remained there for several seconds, head down and shaking it from side to side as I tried to overcome the dizziness and the pain.

"Corazzini said 'shut up'. You must learn to listen." Small-wood's voice was chillingly devoid of all emphasis and inflection. He stood waiting quietly until the last of the passengers had descended or been carried from the tractor cabin, then waved us all into a straight line facing towards Corazzini and himself. Both of them had their backs to the canvas screen, while we were placed just far enough clear of the shelter to be blinded by the increasingly heavy snowfall that swirled down into our eyes, but not so far off as not to be clearly seen by them. Whatever these two did, I was beginning to discover, betrayed that economy of movement and unquestioning sureness of the complete professionals who had long ago worked out the answers to and counters against any of a vast range and permutation of situations they were ever likely to encounter.

Smallwood beckoned me.

"You haven't finished your radio call, Dr Mason. Finish it. Your friend Hillcrest must be wondering at the delay." The gun in his hand came forward a fraction of an inch, just enough for the movement to be perceptible. Tor your own sake, do nothing to arouse his suspicions. Don't be clever. Keep it brief."

I kept it brief. I excused the interruption of transmission on the grounds that Mahler had taken a sudden turn for the worse – as indeed, I thought bitterly, he had – said that I'd guard the missile mechanism with my life and apologised for cutting the call short, but said it was essential to get Mahler to Uplavnik with all speed.

"Finish it off," Smallwood said softly in my ear. I nodded.

"That's the lot then, Captain Hillcrest. Will make the noon schedule. This is Mayday signing off. Mayday, Mayday."

I switched off, and turned indifferently away. I had taken only one step when Smallwood caught my shoulder and whirled me round. For such an apparently slight man, he was phenomenally strong. I gasped as his pistol barrel dug into my stomach.

" 'Mayday', Dr Mason?" he asked silkily. "What is 'Mayday'?"

"Our call-sign, of course," I said irritably.

"Your call-sign is GFK."

"Our call-up is GFK. Our signing-off is 'Mayday'."

"You're lying." I wondered how I could ever have thought this face meek and nervous and colourless. The mouth was a thin hard line, the upper eyelids bar-straight and hooded above the unwinking eyes. Flat marbled eyes of a faded light-blue. A killer's eyes. "You're lying," he repeated.

"I'm not lying," I said angrily.

"Count five and die." His eyes never left mine, the pressure of the gun increased. "One . . . two . . . three – "

"I'll tell you what it is!" The cry came from Margaret Ross. 'Mayday' is the international air distress signal, the SOS... I had to tell him, Dr Mason, I had to!" Her voice was a shaking sob. "He was going to kill you."

"I was indeed," Smallwood agreed. If he felt either anger or apprehension, no trace of either appeared in the calm conversational voice. "I should do it now – you've lost us four hours' head start. But courage happens to be one of the few virtues I admire. . . . You are an extremely brave man, Dr Mason. Your courage is a fair match for your – ah – lack of perspicacity, shall we say."

"You'll never get off the ice-cap, Small wood," I said steadily. "Scores of ships and planes are searching for you, thousands of men. They'll get you and they'll hang you for these five dead men."

"We shall see." He gave a wintry smile, and now that he had removed his rimless glasses I could see that the man's smile left his eyes untouched, left them flat and empty and lifeless, like the stained glass in a church and no sun behind it. "All right, Corazzini, the box. Dr Mason, bring one of the maps from the driver's seat."

"In a moment. Perhaps you would care to explain – "

"Explanations are for children." The voice was level, curt, devoid of all inflection." I'm in a hurry, Dr Mason. Bring the map."

I brought it and when I returned Corazzini was sitting on the front of the tractor sled with a case before him. But it wasn't the leather-covered portable radio: it was Smallwood's robe case.

Corazzini snapped open the catches, pulled out Bible, robes and divinity hood, tossed them to one side then carefully brought out a metal box which looked exactly like a tape-recorder: indeed, when he shone his torch on it I could clearly see the word 'Grundig'. But it soon became apparent that it was like no tape-recorder that I had ever seen.

The twin spools he ripped off the top of the machine and sent spinning away into the darkness and the snow, the tape unwinding in a long convoluted streamer. By this time I would have taken long odds that anyone suspicious enough to investigate would have found that tape perfectly genuine: probably, I thought bitterly, Bach's organ music, in keeping with Smallwood's late ecclesiastical nature.

Still in silence, we watched Corazzini undo and fling away the false top of the recorder, but not before I had time to notice the padded spring clips on its underside – the perfect hiding place for a couple of automatics: revealed now were controls and calibrated dials that bore no resemblance to those of a tape-recorder. Corazzini straightened and erected a hinged telescopic aerial, clamped a set of headphones to his ears, made two switches and started to turn a dial, at the same time watching a green magic eye similar to those found in tape-recorders and many modern radios. Faintly, but unmistakably, I could hear a steady whine coming from the earphones, a whine which altered in pitch and intensity as Corazzini turned the dial. When it reached its maximum strength, he turned his attention to a built-in alcohol compass about three inches in diameter. A few moments later he doffed the earphones and turned round, apparently satisfied.

"Very strong, very clear," he announced to Small wood. "But there's too heavy a deviation factor from all the metal in the tractor and sledge. Back in two minutes. Your torch, Dr Mason."