Over our black sugarless coffee and biscuits I told our passengers that this would be only a brief three-hour halt, that they should try to get what sleep they could: most of them, myself included, were already red-eyed and drooping from exhaustion. Three hours, no more: not often did Greenland offer travel weather like this, and the chance was not to be missed.

Beside me, as I drank my coffee, was Theodore Mahler. He was for some reason restless, ill at ease, jerky and nervous, and his eyes and attention both wandered so much that it was easy enough for me to find out what I wanted.

When my cup was empty, I whispered in Mahler's ear that there was a little matter that I wished to discuss privately with him. He looked at me in surprise, hesitated, then nodded in agreement, rising to follow me as I moved out into the darkness.

A hundred yards away I stopped, switched on my torch so that he blinked in its beam, and slid my Beretta forward until its barrel was clearly visible, sharply outlined in the harsh white glare. I heard the catch of the breath, saw the eyes widening in fear and horror.

"Save the act for the judge, Mahler," I said bleakly. "I'm not interested in it. All I want is your gun."

"My gun?" Mahler had slowly lifted his arms until his hands were at shoulder level, and his voice wasn't quite steady. "I – I don't understand, Dr Mason. I have no gun."

"Naturally." I jerked the barrel of the Beretta to lend emphasis to my words. Turn round."

"What are you going to do? You're making a – "

"Turn round!"

He turned. I took a couple of steps forward, ground the muzzle of the automatic none too gently into the small of his back, and started to search him with my free hand.

He was wearing two overcoats, a jacket, several sweaters and scarves, two-pairs of trousers and layer upon layer of underclothes: searching him was easier said than done. It took me a full minute to convince myself that he wasn't carrying a weapon of any kind. I stepped back, and he came slowly round to face me.

"I hope you're quite satisfied now, Dr Mason?"

"We'11 see what we find in your case. As for the rest, I'm satisfied enough. I have all the proof I want." I dipped the torch beam to illuminate the handful of sugar I'd taken from the pocket of his inner overcoat – there had been well over a pound in either pocket. "You might care to explain where you got this from, Mr Mahler?"

"I don't have to tell you that, do I?" His voice was very low. "I stole it, Dr Mason."

"You did indeed. A remarkably small-time activity for a person who operates on the scale you do. It was just your bad luck, Mahler, that I happened to be looking directly at you when the theft of the sugar was mentioned back in the cabin. It was just your bad luck that when we had our coffee just now it was dark enough for me to have a swig from your cup without your knowledge: it was so stiff with sugar that I couldn't even drink the damn' stuff. Curious, isn't it, Mahler, that such a tiny thing as giving way to a momentary impulse of greed should ruin everything? But I believe it's always the way: the big slip-up never brings the big criminal to book, because he never makes any. If you'd left that sugar alone when you were smashing up the valves, I'd never have known. Incidentally, what did you do with the rest of the sugar? In your grip? Or just thrown away?"

"You're making a very grave mistake, Dr Mason." Mahler's voice was steady now, and if it held any trace of worry or guilt I couldn't detect it. But I was now far beyond the naive stage of expecting to detect anything of the sort. "I didn't touch those valves. And, apart from the few handfuls I took, the sugar bag was quite intact when I left it."

"Of course, of course." I waved the Beretta. "Back to the tractor, my friend, and let's have a look at this case of yours."

"No!"

"Don't be crazy," I snapped. "I have a gun, Mahler. Believe me, I won't hesitate to use it."

"I believe you. I think you would be quite ruthless if the need arose. Oh, I don't doubt you're tough, Doctor, as well as being headstrong, impulsive and not very subtle, but because I rather respect your efficient and selfless handling of an awkward and ugly situation for which you were in no way responsible, I don't want to see you make a complete fool of yourself in public." He lifted his right hand towards the lapel of his coat. "Let me show you something."

I jerked the Beretta forward, but the gesture was quite needless. As he pushed his hand under his topcoats, Mahler's gestures were smooth and unhurried, just as smooth and unhurried when he brought his hand out again and passed over to me a leather-covered card. I stepped back a few feet, flipped open the card and glanced down at it.

That one glance was enough – or should have been enough. I'd seen these cards scores of times before, but I stared down at this one as if I'd never seen one in my life. This was a completely new factor, it knocked all my preconceived notions on the head, and I needed time, time for reorient a tion, for understanding, for quelling the professional fear that came hard on the heels of that understanding. Then, slowly, I folded the card, pulled down my snow-mask, stepped close to Mahler and pulled his down also. In the harsh glare of the torch, his face was blue and white with the cold, and I could see the jutting of the jaw muscles as he clamped his teeth together to keep them from chattering uncontrollably.

"Breathe out," I said.

He did as I asked, and there was no mistaking it, none at all: the sweet acetone breath of the advanced and untreated diabetic can't possibly be confused with anything else. Wordlessly, I handed him back the card and thrust the automatic into my parka pocket.

At last I said quietly: "How long have you had this, Mr Mahler?"

"Thirty years."

"A pretty advanced condition?" When it came to discussing a man's illness with him, I had little time for the professional reticence of many of my colleagues: besides, the average elderly diabetic had survived to that age simply because he was intelligent about the dietary and medical treatment of his trouble, and usually knew all about it.

"My doctor would agree with you." I caught the smile on his face as he pushed his mask up, and there wasn't much humour in it. "So would I."

"Twice daily injections?"

"Twice," he nodded. "Before breakfast and in the evening."

"But don't you carry a hypo and – "

"Normally," he interrupted. "But not this time. The Gander doctor gave me a jab and as I can usually carry on a few hours overdue without Ul effects I thought I'd wait until we got to London." He tapped his breast pocket. "This card's good anywhere."

"Except on the Greenland ice-cap," I said bitterly. "But then I don't suppose you anticipated a stop-over here. What diet were you on?"

"High protein, high starch."

"Hence the sugar?" I looked down at the white crystals still clenched in my left mitten.

"No." He shrugged. "But I know sugar used to be used for the treatment of coma. I thought maybe if I stuffed enough into myself.. . . Well, anyway, you know now why I turned criminal."

"Yes, I know now. My apologies for the gun-waving act, Mr Mahler, but you must admit I had every justification. Why in the hell didn't you tell me before now? I am supposed to be a doctor, you know."

"I would have had to tell you sooner or later, I suppose. But right now you'd plenty of troubles of your own without worrying about mine also. And I didn't think there would be much chance of your carrying insulin among your medical stores."

"We don't – we don't have to. Everybody gets a thorough medical before going on an IGY station, and diabetes hardly develops overnight. . . . You take it all very calmly, I must say, Mr Mahler. Come on, let's get back to the tractor."