His vertical safari had satisfied his need to visit Earth, and he did his best to ignore the various aches and pains acquired there when he returned to his apartment at Level 10,000 – a prestigious location, even in this democratic society. Indra, however, was mildly shocked by his appearance, and ordered him straight to bed.

'Just like Antaeus – but in reverse!' she muttered darkly. 'Who?' asked Poole: there were times when his wife's erudition was a little overwhelming, but he had determined never to let it give him an inferiority complex.

'Son of the Earth Goddess, Gaea. Hercules wrestled with him – but every time he was thrown to the ground, Antaeus renewed his strength.'

'Who won?'

'Hercules, of course – by holding Antaeus in the air, so Ma couldn't recharge his batteries.'

'Well, I'm sure it won't take me long to recharge mine. And I've learned one lesson. If I don't get more exercise, I may have to move up to Lunar Gravity level.'

Poole's good resolution lasted a full month: every morning he went for a brisk five-kilometre walk, choosing a different level of the Africa Tower each day. Some floors were still vast, echoing deserts of metal which would probably never be occupied, but others had been landscaped and developed over the centuries in a bewildering variety of architectural styles. Many were borrowings from past ages and cultures; others hinted at futures which Poole would not care to visit. At least there was no danger of boredom, and on many of his walks he was accompanied, at a respectful distance, by small groups of friendly children. They were seldom able to keep up with him for long.

One day, as Poole was striding down a convincing – though sparsely populated – imitation of the Champs Elyse´es, he suddenly spotted a familiar face.

'Danil!' he called.

The other man took not the slightest notice, even when Poole called again, more loudly.

'Don't you remember me?'

Danil – and now that he had caught up with him, Poole did not have the slightest doubt of his identity – looked genuinely baffled.

'I'm sorry,' he said. 'You're Commander Poole, of course. But I'm sure we've never met before.'

Now it was Poole's turn to be embarrassed.

'Stupid of me,' he apologized. 'Must have mistaken you for someone else. Have a good day.'

He was glad of the encounter, and was pleased to know that Danil was back in normal society. Whether his original crime had been axe-murders or overdue library books should no longer be the concern of his one-time employer; the account had been settled, the books closed. Although Poole sometimes missed the cops-and-robbers dramas he had often enjoyed in his youth, he had grown to accept the current wisdom: excessive interest in pathological behaviour was itself pathological.

With the help of Miss Pringle, Mk III, Poole had been able to schedule his life so that there were even occasional blank moments when he could relax and set his Braincap on Random Search, scanning his areas of interest. Outside his immediate family, his chief concerns were still among the moons of Jupiter/Lucifer, not least because he was recognized as the leading expert on the subject, and a permanent member of the Europa Committee.

This had been set up almost a thousand years ago, to consider what, if anything, could and should be done about the mysterious satellite. Over the centuries, it had accumulated a vast amount of information, going all the way back to the Voyager flybys of 1979 and the first detailed surveys from the orbiting Galileo spacecraft of 1996.

Like most long-lived organizations, the Europa Committee had become slowly fossilized, and now met only when there was some new development. It had woken up with a start after Halman's reappearance, and appointed an energetic new chairperson whose first act had been to co-opt Poole.

Though there was little that he could contribute that was not already recorded, Poole was very happy to be on the Committee. It was obviously his duty to make himself available, and it also gave him an official position he would otherwise have lacked. Previously his status was what had once been called a 'national treasure', which he found faintly embarrassing. Although he was glad to be supported in luxury by a world wealthier than all the dreams of war-ravaged earlier ages could have imagined, he felt the need to justify his existence.

He also felt another need, which he seldom articulated even to himself. Halman had spoken to him, if only briefly, at their strange encounter two decades ago. Poole was certain that, if he wished, Halman could easily do so again. Were all human contacts no longer of interest to him? He hoped that was not the case; yet that might be one explanation of his silence.

He was frequently in touch with Theodore Khan – as active and acerbic as ever, and now the Europa Committee's representative on Ganymede. Ever since Poole had returned to Earth, Ted had been trying in vain to open a channel of communication with Bowman. He could not understand why long lists of important questions on subjects of vital philosophical and historic interest received not even brief acknowledgements.

'Does the Monolith keep your friend Halman so busy that he can't talk to me?' he complained to Poole. 'What does he do with his time, anyway?'

It was a very reasonable question; and the answer came, like a thunderbolt out of a cloudless sky, from Bowman himself – as a perfectly commonplace vidphone call.

'Hello, Frank. This is Dave. I have a very important message for you. I assume that you are now in your suite in Africa Tower. If you are there, please identify yourself by giving the name of our instructor in orbital mechanics. I will wait for sixty seconds, and if there is no reply will try again in exactly one hour.'

That minute was hardly long enough for Poole to recover from the shock. He felt a brief surge of delight, as well as astonishment, before another emotion took over. Glad though he was to hear from Bowman again, that phrase 'a very important message' sounded distinctly ominous.

At least it was fortunate, Poole told himself, that he's asked for one of the few names I can remember. Yet who could forget a Scot with a Glasgow accent so thick it had taken them a week to master it? But he had been a brilliant lecturer – once you understood what he was saying.

'Dr Gregory McVitty.'

'Accepted. Now please switch on your Braincap receiver. It will take three minutes to download this message. Do not attempt to monitor: I am using ten-to-one compression. I will wait two minutes before starting.'

How is he managing to do this? Poole wondered. Jupiter/Lucifer was now over fifty light-minutes away, so this message must have left almost an hour ago. It must have been sent with an intelligent agent in a properly addressed package on the Ganymede-Earth beam – but that would have been a trivial feat to Halman, with the resources he had apparently been able to tap inside the Monolith.

The indicator light on the Brainbox was flickering. The message was coming through.

At the compression Halman was using, it would take half an hour for Poole to absorb the message in real-time. But he needed only ten minutes to know that his peaceful life-style had come to an abrupt end