Poole never had much contact with the team who assembled the weapon everyone hoped would never have to be used. The operation – ominously, but aptly, named Damocles – was so highly specialized that he could contribute nothing directly, and he saw enough of the task force to realize that some of them might almost belong to an alien species. Indeed, one key member was apparently in a lunatic asylum – Poole had been surprised to find that such places still existed – and Chairperson Oconnor sometimes suggested that at least two others should join him.

'Have you ever heard of the Enigma Project?' she remarked to Poole, after a particularly frustrating session. When he shook his head, she continued: 'I'm surprised – it was only a few decades before you were born: I came across it while when I was researching material for Damocles. Very similar problem – in one of your wars, a group of brilliant mathematicians was gathered together, in great secrecy, to break an enemy code... incidentally, they built one of the very first real computers, to make the job possible.'

'And there's a lovely story – I hope it's true – that reminds me of our own little team. One day the Prime Minister came on a visit of inspection, and afterwards he said to Enigma's Director: "When I told you to leave no stone unturned to get the men you needed, I didn't expect you to take me so literally".'

Presumably all the right stones had been turned for Project Damocles. However, as no one knew whether they were working against a deadline of days, weeks or years, at first it was hard to generate any sense of urgency. The need for secrecy also created problems; since there was no point in spreading alarm throughout the Solar System, not more than fifty people knew of the project. But they were the people who mattered – who could marshal all the forces necessary, and who alone could authorize the opening of the Pico Vault, for the first time in five hundred years.

When Halman reported that the Monolith was receiving messages with increasing frequency, there seemed little doubt that something was going to happen. Poole was not the only one who found it hard to sleep in those days, even with the help of the Braincap's anti-insomnia programs. Before he finally did get to sleep, he often wondered if he would wake up again. But at last all the components of the weapon were assembled – a weapon invisible, untouchable and unimaginable to almost all the warriors who had ever lived.

Nothing could have looked more harmless and innocent than the perfectly standard terabyte memory tablet, used with millions of Braincaps every day. But the fact that it was encased in a massive block of crystalline material, criss-crossed with metal bands, indicated that it was something quite out of the ordinary. Poole received it with reluctance; he wondered if the courier who had been given the awesome task of carrying the Hiroshima atom bomb's core to the Pacific airbase from which it was launched had felt the same way. And yet, if all their fears were justified, his responsibility might be even greater.

And he could not be certain that even the first part of his mission would be successful. Because no circuit could be absolutely secure, Halman had not yet been informed about Project Damocles; Poole would do that when he returned to Ganymede.

Then he could only hope that Halman would be willing to play the role of Trojan Horse – and, perhaps, be destroyed in the process.

It was strange to be back in the Hotel Grannymede after all these years – strangest of all, because it seemed completely unchanged, despite everything that had happened. Poole was still greeted by the familiar image of Bowman as he walked into the suite named after him: and, as he expected, Bowman/Halman was waiting, looking slightly less substantial than the ancient hologram.

Before they could even exchange greetings, there was an interruption that Poole would have welcomed – at any other time than this. The room vidphone gave its urgent trio of rising notes – also unchanged since his last visit -and an old friend appeared on the screen.

'Frank!' cried Theodore Khan, 'why didn't you tell me you were coming! When can we meet? Why no video – someone with you? And who were all those official-looking types who landed at the same time -'

'Please Ted! Yes, I'm sorry – but believe me, I've got very good reasons – I'll explain later. And I do have someone with me – call you back just as soon as I can. Good-bye!'

As he belatedly gave the 'Do Not Disturb' order, Poole said apologetically: 'Sorry about that – you know who it was, of course.'

'Yes – Dr Khan. He often tried to get in touch with me.'

'But you never answered. May I ask why?' Though there were far more important matters to worry about, Poole could not resist putting the question.

'Ours was the only channel I wished to keep open. Also, I was often away. Sometimes for years.'

That was surprising – yet it should not have been. Poole knew well enough that Halman had been reported in many places, in many times. Yet – 'away for years'? He might have visited quite a few star systems – perhaps that was how he knew about Nova Scorpio, only forty light-years distant. But he could never have gone all the way to the Node; there and back would have been a nine-hundred-year journey.

'How lucky that you were here when we needed you!' It was very unusual for Halman to hesitate before replying. There was much longer than the unavoidable three-second time-lag before he said slowly 'Are you sure that it was luck?'

'What do you mean?'

'I do not wish to talk about it, but twice I have – glimpsed – powers – entities – far superior to the Monoliths, and perhaps even their makers. We may both have less freedom than we imagine.'

That was indeed a chilling thought; Poole needed a deliberate effort of will to put it aside and concentrate on the immediate problem.

'Let us hope we have enough free-will to do what is necessary. Perhaps this is a foolish question. Does the Monolith know that we are meeting? Could it be – suspicious?'

'It is not capable of such an emotion. It has numerous fault-protection devices, some of which I understand. But that is all.'

'Could it be overhearing us now?'

'I do not believe so.'

I wish that I could be sure it was such a naïve and simple-minded super-genius, thought Poole as he unlocked his briefcase and took out the sealed box containing the tablet. In this low gravity its weight was almost negligible; it was impossible to believe that it might hold the destiny of Mankind.

'There was no way we could be certain of getting a secure circuit to you, so we couldn't go into details. This tablet contains programs which we hope will prevent the Monolith from carrying out any orders which threaten Mankind. There are twenty of the most devastating viruses ever designed on this, most of which have no known antidote; in some cases, it is believed that none is possible. There are five copies of each. We would like you to release them when – and if – you think it is necessary. Dave – Hal – no one has ever been given such a responsibility. But we have no other choice.'

Once again, the reply seemed to take longer than the three-second round trip from Europa.

'If we do this, all the Monolith's functions may cease. We are uncertain what will happen to us then.'

'We have considered that, of course. But by this lime, you must surely have many facilities at your command -some of them probably beyond our understanding. I am also sending you a petabyte memory tablet. Ten to the fifteenth bytes is more than sufficient to hold all the memories and experiences of many lifetimes. This will give you one escape route: I suspect you have others.'