And now Harry was back in Leipzig, standing by Mobius's grave as before. Almost nine years had passed since last he was here, but he hadn't forgotten those events which terminated his first visit. And so on this occasion he'd come by night.

A moon hung low over the city's skyline, and the stars were very bright between streamers of fast-fleeing cloud. The night wind, moaning through the headstones, sent wrinkled leaves scurrying like mice, and Harry felt a chill in his bones which was born partly of the natural cold of a November night, and partly of his feeling of alienation here in this place. But the cemetery gates were closed for the night, the lights in the city subdued, and apart from the scrape of leaves all was silence.

He sought Mobius out and found him, and as before the great mathematician was busy with his formulae and his calculations. Tables of planetary mass and motion, the 'weights' of the sun and her satellite worlds in their careening round, were balanced against orbital velocities and gravitic forces; formulae so complex that even Harry's intuitive grasp found their purpose elusive, together with simultaneous equations whose answers filled themselves in even as he watched; all of these figures and configurations beat on Harry's awareness like the ever-changing results of an on-going process on the screen of some vast computer. And Harry saw that the problem was so complex and so close to completion that he let it go on undisturbed by his presence to the end. At which time the screen went blank and Mobius sighed. It was a strange thing, even now, to hear the 'sigh' of a dead man.

'Sir?' said Harry. 'Are you available now?'

'Eh?' said Mobius, in that moment before he recognized Harry's thoughts. Then: 'Is that you, Harry?' he continued eagerly. 'I thought there was someone here. You very nearly put me off just then, and I was working on something which is very important!'

'I know,' Harry nodded. 'I saw it, but I didn't want to disturb you. Those are very wonderful discoveries!'

'Oh?' Mobius seemed surprised. 'You could understand my working, then? Very well, and what have I discovered?'

Harry drew back a little, hesitating. He was in the presence of genius and he knew it. Mobius had been a great mathematician all his life, and after that life he had continued his work unabated. Where Harry's mathematical skills were intuitive, Mobius had worked hard to achieve his results. No quantum leaps for him but dogged trial and error and an unwavering, all-consuming passion for his subject. It seemed somehow improper for Harry to have come here at this time, spying on the man in his triumph.

'Not at all,' Mobius tut-tutted him. 'What? - a man who can impose his physical being on the metaphysical universe, and use it at will? Spying on me? I consider you a colleague, Harry, an equal! And truth be told, you couldn't have come visiting at a more opportune time. Now come on, tell me what I've been doing. What is it that I've proved with my numbers, eh?'

Harry shrugged. 'Very well,' he said. 'You've shown that instead of the nine planets we believed to exist in the solar system, there are in fact eleven. Both of the new worlds are small, but true planets for all that. One occupies a position exactly behind Jupiter, with the same rotation period, so that it's always occluded, and the other's a non-reflector and lies about as far out again as Pluto from the sun.'

'Good!' Mobius applauded him. 'And their moons?'

'Eh?' Harry was taken by surprise. 'I read only the problem you'd set yourself and the answers to the problem as you arrived at them! There were slight deviations -percentages of error, I suppose - but...' He paused.

'But? But?' Harry could almost picture Mobius raising his eyebrows. 'All the clues were there in the equations, Harry. No? Very well. I'll tell you:

The inner world has no moon, but the "percentage of error", as you call it, for the outer world was just too big to be ignored. I have checked it and it indicates an almost spherical nickel-iron moon three kilometres in diameter orbiting the parent at a distance of twenty-four thousand planetary circumferences. Now that is what we call a calculation! Of course, I shall prove it by going there and seeing it for myself.'

Harry shook his head in defeat, offered a wry grimace. 'You're too good for me,' he said. 'You always will be.' And after a moment: 'Do you want me to let this "leak out", as it were? I could do that easily enough, with just sufficient information to set the entire astronomical fraternity jumping! It could be done anonymously, by an "amateur", you understand, on the solemn promise that when the calculations are shown to be correct, then one of the two worlds should be named Mobius!' Mobius was stunned. 'Could you really do that, Harry?' 'I'm sure I could find a way.'

'My boy... God!' Mobius was overjoyed at the prospect. 'Harry, how I wish I could shake your hand!'

'You can do rather more than that,' Harry told him, growing serious in a moment. 'You remember the last time I came to see you I had a problem? Well, now I have an even bigger one.'

'Let's have it, then,' said the other at once, and Harry told him of his quest for his wife and son. He finished by explaining:

'And so you see, it's no longer simply a question of my family, but I also have the British agent Michael Simmons to consider.'

Mobius seemed nonplussed. 'And you've come to me for help? Well, obviously you have - but for the life of me I can't see what I can do! I mean, if they're not here, these three people - if they have physically ceased to exist in this universe - then how can I or anyone else suggest where or how to find them? The universe is The Universe, Harry. Its very name defines it. It is THE ALL. If they're not in it, then they're not anywhere.'

'That was my line of reasoning, too.' Harry admitted, ' - until recently. But you and me, why, don't we both contradict that very fact?'

'Eh? How's that?'

'The Mobius Continuum,' Harry answered, by way of explanation. 'You yourself admit that it's a purely metaphysical plane, not of this universe. Step into the Mobius Continuum and you step out of the three mundane dimensions. The Mobius Continuum not only transcends the three dimensions of mundane space but time also, and runs parallel to all of them! And what of a black hole?'

'What of it?' (Mobius's mental shrug.)

'Well, isn't a black hole an exit from this universe? That's how they've always been explained to me: a focus of gravity so great that space and time themselves are drawn into the whorl. And if they are exits from the here and now, then where the hell do they lead?'

To another part of the universe,' Mobius answered. That seems the only likely explanation to me. Mind you, I haven't really looked at black holes yet. I have them scheduled, though.'

'Are you missing the point or deliberately avoiding it?' Harry wanted to know. This is my question: if a black hole goes somewhere, emerging maybe light-years away, what of the space in between? Where is the material which is drawn into the hole, between its disappearing and its reappearing? You see, to me this all seems very much like our Mobius Continuum.'

'Go on,' Mobius was fascinated.

'OK,' said Harry, 'let's look at it this way. First we have the ... let's call it the mundane universe. And we'll say it looks like this:'

He showed Mobius a mental diagram.

'Why the bends?' the mathematician was immediately curious.

'Because without them it would just be a pair of straight lines,' Harry told him. 'The bends give it definition, make it look like something.'

'Like a ribbon?'

'For the purpose of the exercise, why not? For all I know it could be a circle, or maybe a sphere. But this way we can envisage a past and a future, too.'