They spoke perfect Basque – an excruciatingly difficult tongue with no affinity to any other known language of mankind. Clearly, the space visitors were remarkable linguists, even if their geography was oddly deficient.

So it went on, case after case. Very few of the contactees were actually lying or insane; most of them sincerely believed their own stories, and retained that belief even under hypnosis. And some were just victims of practical jokes or improbable accidents – like the unlucky amateur archaeologists who found the props that a celebrated science-fiction moviemaker had abandoned in the Tunisian desert almost four decades earlier.

Yet only at the beginning – and at the very end – was any human being genuinely aware of his presence; and that was because he so desired it.

The world was his to explore and examine as he pleased, without restraint or hindrance. No walls could keep him out, no secrets could be hidden from the senses he possessed. At first he believed that he was merely fulfilling old ambitions, by visiting the places he had never seen in that earlier existence. Not until much later did he realize that his lightning-like sallies across the face of the globe had a deeper purpose.

In some subtle way, he was being used as a probe, sampling every aspect of human affairs. The control was so tenuous that he was barely conscious of it; he was rather like a hunting dog on a leash, allowed to make excursions of his own, yet nevertheless compelled to obey the overriding wishes of his master.

The pyramids, the Grand Canyon, the moon-washed snows of Everest – these were choices of his own. So were some art galleries and concert halls; though he would certainly, on his own initiative, never have endured the whole of the Ring.

Nor would he have visited so many factories, prisons, hospitals, a nasty little war in Asia, a racecourse, a complicated orgy in Beverly Hills, the Oval Room of the White House, the Kremlin archives, the Vatican Library, the sacred Black Stone of the Kaabah at Mecca.

There were also experiences of which he had no clear memory, as if they had been censored – or he was being protected from them by some guardian angel. For example -What was he doing at the Leakey Memorial Museum, in Olduvai Gorge? He had no greater interest in the origin of Man than any other intelligent member of the species H. sapiens, and fossils meant nothing to him. Yet the famous skulls, guarded like crown jewels in their display cases, aroused strange echoes in his memory, and an excitement for which he was unable to account. There was a feeling of déja vu stronger than any he had ever known; the place should be familiar – but something was wrong. It was like a house to which one returns after many years, to find that all the furniture has been changed, the walls moved, and even the stairways rebuilt.

It was bleak, hostile terrain, dry and parched. Where were the lush plains and the myriad fleet-footed herbivores that had roamed across them, three million years ago?

Three million years. How had he known that?

No answer came from the echoing silence into which he had thrown the question. But then he saw, once more looming before him, a familiar black rectangular shape. He approached, and a shadowy image appeared in its depths, like a reflection in a pool of ink.

The sad and puzzled eyes that stared back from beneath that hairy, receding forehead looked beyond him into a future they could never see. For he was that future, a hundred thousand generations further down the stream of time.

History had begun there; that at least he now understood. But how – and above all, why – were secrets still withheld from him?

But there was one last duty, and that was hardest of all. He was still sufficiently human to put it off until the very end.

Now what's she up to? the duty nurse asked herself, zooming the TV monitor onto the old lady. She's tried lots of tricks, but this is the first time I've seen her talking to her hearing aid, for goodness sake. I wonder what she's saying?

The microphone was not sensitive enough to pick up the words, but that scarcely seemed to matter. Jessie Bowman had seldom looked so peaceful and content. Though her eyes were closed, her entire face was wreathed in an almost angelic smile while her lips continued to form whispered words.

And then the watcher saw something that she tried hard to forget because to report it would instantly disqualify her in the nursing profession. Slowly and jerkily, the comb lying on the bedside table raised itself in the air as if lifted by clumsy, invisible fingers.

On the first attempt, it missed; then, with obvious difficulty, it began to part the long silver strands, pausing sometimes to disentangle a knot.

Jessie Bowman was not speaking now, but she continued to smile. The comb was moving with more assurance, and no longer in abrupt, uncertain jerks.

How long it lasted the nurse could never be certain. Not until the comb was gently replaced on the table did she recover from her paralysis.

Ten-year-old Dave Bowman had finished the chore which he always hated but which his mother loved. And a David Bowman who was now ageless had gained his first control of obdurate matter.

Jessie Bowman was still smiling when the nurse finally came to investigate. She had been too scared to hurry; but it would have made no difference anyway.

35 – Rehabilitation

The uproar of Earth was comfortably muted, across the millions of kilometres of space. Leonov's crew watched, with fascination yet with a certain detachment, the debates in the United Nations, the interviews with distinguished scientists', the theorizing of the news commentators, the matter-of-fact yet wildly conflicting accounts of the UFO contactees. They could contribute nothing to the brouhaha, for they had witnessed no further manifestations of any kind. Zagadka, alias Big Brother, remained as blankly indifferent to their presence as ever. And that was indeed an ironic situation; they had come all the way from Earth to solve a mystery – and it looked as if the answer might be right back at their starting point.

For the first time, they felt grateful for the slow velocity of light, and the two-hour delay that made live interviews impossible on the Earth-Jupiter circuit. Even so, Floyd was badgered by so many media requests that he finally went on strike. Nothing more remained to be said, and he had said it at least a dozen times.

Besides, there was still much work to be done. Leonov had to be prepared for the long journey home, so that it would be ready to depart immediately when the launch window opened. The timing was not at all critical; even if they missed by a month, that would merely prolong the trip. Chandra, Curnow, and Floyd would not even notice as they slept their way toward the Sun; but the rest of the crew was grimly determined to leave just as soon as the laws of celestial mechanics permitted.

Discovery still posed many problems. The ship had barely sufficient propellant for the return to Earth, even if it left much later than Leonov and flew a minimum-energy orbit – which would take almost three years. And this would be possible only if Hal could be reliably programmed to carry out the mission with no human intervention except long-range monitoring. Without his cooperation, Discovery would have to be abandoned once again.

It had been fascinating – indeed, deeply moving – to watch the steady regrowth of Hal's personality, from brain-damaged child to puzzled adolescent and at length to slightly condescending adult. Although he knew that such anthropomorphic labels were highly misleading, Floyd found it quite impossible to avoid them.

And there were times when he felt that the whole situation had a haunting familiarity. How often he had seen videodramas in which disturbed youngsters were straightened out by all-wise descendants of the legendary Sigmund Freud! Essentially the same story was being played out in the shadow of Jupiter.