now, the right spur. The prod that will make you take the final step.'

Harry gave a mental nod of acknowledgement. 'You're probably right,' he said. 'I know I will do it; it's just that I ... haven't tried yet? It's something like giving up smoking: you can but can't. You probably will when it's too late, when you're dying of cancer. Except I don't want to wait that long! I mean, I have all the maths, all the theory - I have all the ego, really, the intuition - but I haven't the need, not yet. Or the spur, if you like. Let me tell you what it feels like:

'I'm sitting in a well-lighted room with a window and a door. I look out the window and it's dark out there. It always will be. Not night but a stronger darkness that will last for ever. It's the darkness of the spaces between the spaces. I know there are other rooms out there some­where. My problem is that I don't have any directions. If I go out that door I'll be part of the darkness, surrounded by it. I might not be able to come in again, here or anywhere else. It's not so much that I can't go out but more that I don't want to think about what it's like out there. Actually, to know it's there is to know I can go out into it. I feel that the going will just be an extension of the other things I can do, but an untried extension. I'm a chicken in a shell, and I won't break out until I have to!'

'Who are you talking to, Mr Harry Keogh?' asked a voice that wasn't Mobius', a flat, cold voice, as curious as it was emotionless.

'What?' Startled, Harry looked up.

There were two of them, and it was obvious who or what they were. Even knowing nothing about spying or East-West politics, he would have recognised these two on sight. They chilled him more than the thin wind which now began to keen through the empty cemetery, blowing dead leaves and scraps of paper along the aisles between the tombs.

One was very tall, the other short, but their dark-grey overcoats, their hats pulled down at the front and their narrow-rimmed spectacles were so uniform in themselves as to make them appear twins. Certainly twins in their natures, in their thoughts and their petty ambitions. As plain-clothes men - policemen, probably political - they were quite unmistakable.

'What?' Harry said again, coming stiffly to his feet. 'Was I talking to myself again? Fm sorry about that, I do it all the time. It's just a habit of mine.'

Talking to yourself?' the tall one repeated him, and shook his head. 'No, I don't think so.' His accent was thick, his lips thin as his mirthless smile. 'I think you were talking to someone else - probably to another spy, Harry Keogh!'

Harry backed away from them a pace or two. 'I really don't know what -' he began.

'Where is your radio, Mr Keogh?' said the short one. He came forward, kicked at the dirt of the grave where Harry had been sitting. 'Is it here, buried in the soil, perhaps? Day after day, sitting here, talking to yourself? You must think we're all fools!'

'Listen,' Harry croaked, still backing away. 'You must have the wrong man. Spy? That's crazy. I'm a tourist, that's all.'

'Oh?' said the tall one. 'A tourist? In the middle of winter? A tourist who comes and sits in the same grave­yard day after day, to talk to himself? You can do better than that, Mr Keogh. And so can we. We have it on good authority that you are a British agent, also that you're a murderer. So now, please, you will come with us.'

'Don't go with them, Harry!' it was Keenan Gormley's voice, coming from nowhere, unbidden to Harry's mind. 'Run, man, run!'

'What?' Harry gasped. 'Keenan? But how...?'

'Oh, Harry! My Harry!' cried his mother. 'Please be careful!'

'What?' he said again, shaking his head, still backing away from the two men.

The small one produced handcuffs, said: 'I must warn you, Mr Keogh, against resistance. We are counter­espionage officials of the Grenzpolizei, and -'

'Hit him, Harry!' urged Graham 'Sergeant' Lane in Harry's innermost ear. 'You have the measure of both these lads. You know the way. Do it to them before they do it to you. But watch it - they're armed!'

As the short one took three quick paces forward, holding out the handcuffs, Harry adopted a defensive stance. Also closing in, the tall one yelled: 'What's this? You threaten violence? You should know, Harry Keogh, that our orders are to take you dead or alive!'

The short one made to snap the cuffs on Harry's wrists. At the last moment Harry slapped them aside, half-turned, lashed out with his heel at the end of a leg stiffened into a bar of solid bone. The blow took the short one in the chest, snapped ribs, drove him backwards into his tall colleague. Screaming his agony, he slipped to the ground.

'You can't win, Harry!' Gormley insisted. 'Not like this.'

'He's right,' said James Gordon Hannant. 'This is your last chance, Harry, and you have to take it. Even if you stop these two there'll be others. This isn't the way. You have to use your talent, Harry. Your talent is bigger than you suspect. I didn't teach you anything about maths - I only showed you how to use what was in you. But your full potential remains untapped. Man, you have formulae I haven't even dreamed of! You yourself once said some­thing like that to my son, remember?'

Harry remembered.

Strange equations suddenly flashed on the screen of his

mind. Doors opened where no doors should be. His metaphysical mind reached out and grasped the physical world, eager to bend it to his will. He could hear the felled plain-clothes man screaming his rage and pain, could see the taller one reaching into his overcoat and drawing out an ugly, short-barrelled weapon. But printed over this picture of the real world, the doors in the Mobius space-time dimension were there within reach, their dark thresholds seeming to beckon.

'That's it, Harry!' cried Mobius himself. 'Any one of them will do!'

'I don't know where they go!' he yelled out loud.

'Good luck, Harry!' shouted Gormley, Hannant and Lane, almost in unison.

The gun in the tall agent's hand spouted fire and lead. Harry twisted, felt a hot breath against his neck as something snatched angrily at the collar of his coat. He whirled, leaped, drop-kicked the tall man and felt deep satisfaction as his feet crashed into face and shoulder. The man went down, his weapon clattering to the hard ground. Cursing and spitting blood and teeth, he scrambled after it, grasped it in two hands, came up into a stumbling crouch.

Out of the corner of his eye, Harry spied a door in the Mobius strip. It was so close that if he reached out his hand he could touch it. The tall agent snarled something incomprehensible, swung his gun in Harry's direction. Harry knocked it aside, grabbed the man's sleeve, tugged him off balance and swung him -

- Through the open door.

The German agent was ... no longer there! From nowhere, an awful, lingering, slowly fading scream came echoing back. It was the cry of the damned, of a soul lost for ever in ultimate darkness.

Harry listened to that cry and shuddered - but only for a moment. Over and above it as it dwindled, he heard

shouted instructions, the crunch of running feet on gravel. Men were coming, dodging between the tombstones, converging on him. He knew that if he was going to use the doors, it had to be now. The injured agent on the ground was holding a gun in hands that trembled like jelly. His eyes were impossibly round for he had seen... something! He was no longer sure if he dared pull the trigger and shoot at this man.

Harry didn't give him time to think it over. Kicking his gun away, he paused for one last split-second and let the screens in his mind display once more their fantastic formulae. The running men were closer; a bullet whined where it struck sparks from marble.