'Fifteen minutes to ignition. All systems nominal.'

'Thank you, Hal.'

'By the way,' said Vasili, from the other ship, 'we're catching up with the Great Black Spot again. Wonder if we can see anything new.'

I rather hope not, thought Curnow; we've got quite enough on our hands at the moment. Nevertheless, he gave a quick glance at the image Vasili was transmitting on the telescope monitor.

At first he could see nothing except the faintly glimmering nightside of the planet; then he saw, on the horizon, a foreshortened circle of deeper darkness. They were rushing toward it with incredible speed.

Vasili increased the light amplification, and the entire image brightened magically. At last, the Great Black Spot resolved itself into its myriad identical elements.

My God, thought Curnow, I just don't believe it!

He heard exclamations of surprise from Leonov: all the others had shared in the same revelation at the same moment.

'Dr Chandra,' said Hal, 'I detect strong vocal stress patterns. Is there a problem?'

'No, Hal,' Chandra answered quickly. 'The mission is proceeding normally. We've just had rather a surprise – that's all. What do you make of the image on monitor circuit 16?'

'I see the nightside of Jupiter. There is a circular area, 3,250 kilometres in diameter, which is almost compietely covered with rectangular objects.'

'How many?'

There was the briefest of pauses, before Hal flashed the number on the video display:

1,355,000 ± 1,000

'And do you recognize them?'

'Yes. They are identical in size and shape to the object you refer to as Big Brother. Ten minutes to ignition. All systems nominal.'

Mine aren't, thought Curnow. So the damn thing's gone down to Jupiter – and multiplied. There was something simultaneously comic and sinister about a plague of black monoliths; and to his puzzled surprise, that incredible image on the monitor screen had a certain weird familiarity.

Of course – that was it! Those myriad, identical black rectangles reminded him of – dominoes. Years ago, he had seen a video documentary showing how a team of slightly crazy Japanese had patiently stood a million dominoes on end, so that when the very first one was toppled, all the others would inevitably follow. They had been arranged in complex patterns, some underwater, some up and down little stairways, others along multiple tracks so that they formed pictures and patterns as they fell. It had taken weeks to set them up; Curnow remembered now that earthquakes had several times foiled the enterprise, and the final toppling, from first domino to the last, had taken more than an hour.

'Eight minutes to ignition. All systems nominal. Dr Chandra – may I make a suggestion?'

'What is it, Hal?'

'This is a very unusual phenomenon. Do you not think I should abort the countdown, so that you can remain to study it?'

Aboard Leonov, Floyd started to move quickly toward the bridge. Tanya and Vasili might be needing him. Not to mention Chandra and Curnow – what a situation! And suppose Chandra took Hal's side? If he did – they might both be right! After all, was this not the very reason they had come here?

If they stopped the countdown, the ships would loop around Jupiter and be back at precisely the same spot in nineteen hours. A nineteen-hour hold would create no problems; if it was not for that enigmatic warning, he would have strongly recommended it himself.

But they had very much more than a warning. Below them was a planetary plague spreading across the face of Jupiter. Perhaps they were indeed running away from the most extraordinary phenomenon in the history of science. Even so, he preferred to study it from a safer distance.

'Six minutes to ignition,' said Hal. 'All systems nominal. I am ready to stop the countdown if you agree. Let me remind you that my prime directive is to study everything in Jupiter space that may be connected with intelligence.'

Floyd recognized that phrase all too well: he had written it himself. He wished he could delete it from Hal's memory.

A moment later, he had reached the bridge and joined the Orlovs. They both looked at him with alarmed concern.

'What do you recommend?' asked Tanya swiftly.

'It's up to Chandra, I'm afraid. Can I speak to him – on the private line?'

Vasili handed over the microphone.

'Chandra? I assume that Hal can't hear this?'

'Correct, Dr Floyd.'

'You've got to talk quickly. Persuade him that the countdown must continue, that we appreciate his – er, scientific enthusiasm – ah, that's the right angle – say we're confident that he can do the job without our help. And we'll be in touch with him all the time, of course.'

'Five minutes to ignition. All systems nominal. I am still waiting for your answer, Dr Chandra.'

So are we all, thought Curnow, only a metre away from the scientist. And if I do have to push that button at last, it will be something of a relief. In fact, I'll rather enjoy it.

'Very well, Hal. Continue the countdown. I have every confidence in your ability to study all phenomena in Jupiter space, without our supervision. Of course, we will be in touch with you at all times.'

'Four minutes to ignition. All systems nominal. Propellant-tank pressurization completed. Voltage steady on plasma trigger. Are you sure you are making the right decision, Dr Chandra? I enjoy working with human beings and have a stimulating relationship with them. Ship's attitude correct to point one milliradian.'

'We enjoy working with you, Hal. And we will still be doing so, even if we are millions of kilometres away.'

'Three minutes to ignition. All systems nominal, Radiation shielding checked. There is a problem of the time lag, Dr Chandra. It may be necessary to consult each other without any delay.'

This is insane, Curnow thought, his hand now never far from the cut-off switch. I really believe that Hal is – lonely. Is he mimicking some part of Chandra's personality that we never suspected?

The lights flickered, so imperceptibly that only someone familiar with every nuance of Discovery's behaviour would have noticed. It could be good news or bad – the plasma firing sequence starting, or being terminated...

He risked a quick glance at Chandra; the little scientist's face was drawn and haggard, and for almost the first time Curnow felt real sympathy for him as another human being. And he remembered the startling information that Floyd had confided in him – Chandra's offer to stay with the ship, and keep Hal company on the three-year voyage home. He had heard no more of the idea, and presumably it had been quietly forgotten after the warning. But perhaps Chandra was being tempted again; if he was, there was nothing that he could do about it at that stage. There would be no time to make the necessary preparations, even if they stayed on for another orbit and delayed their departure beyond the deadline. Which Tanya would certainly not permit after all that had now happened.

'Hal,' whispered Chandra, so quietly that Curnow could scarcely hear him. 'We have to leave. I don't have time to give you all the reasons, but I can assure you it's true.'

'Two minutes to ignition. All systems nominal. Final sequence started. I am sorry that you are unable to stay. Can you give me some of the reasons, in order of importance?'

'Not in two minutes, Hal. Proceed with the countdown. I will explain everything later. We still have more than an hour together.'

Hal did not answer. The silence stretched on and on. Surely the one-minute announcement was overdue ...

Curnow glanced at the clock. My God, he thought, Hal's missed it! Has he stopped the countdown?

Curnow's hand fumbled uncertainly for the switch. What do I do now? I wish Floyd would say something, dammit, but he's probably afraid of making things worse...