'And now he's practically a billionaire,' said Raine.

'He does seem to be on his way to that dubious status.'

'Because he's invented artificial intelligence?' said Ace.

The Doctor frowned thoughtfully. 'Well, this so-called Canterbury AI which he's developed is something called a black box Artificial Intelligence.'

'That doesn't sound at all sinister,' said Raine, suddenly hugging her elbows close to her body as if she was cold, although it was a warm and sunny day. 'Not one little bit.'

'It simply means that no one knows how his software works. You put a question into it, and out comes an answer invariably correct. But how it arrives at the answer is a mystery. It certainly displays all the qualities of intelligence, despite this obdurate opacity. In a sense it's a cla.s.sic example of the Turing test.' The Doctor smiled and set off at a brisk pace, in the opposite direction to Raymond Luthier, his bodyguards, and the ma.s.s of protesters. Ace and Raine followed, glancing back the way they'd come in surprise.

'Aren't we listening to his talk, then?' said Ace.

'No. In fact, we are going to take advantage of the distraction caused by his visit to break into the university administration office and obtain certain useful information.'

'Couldn't we get this information from a computer?' said Ace. 'Unfortunately, not. It's extremely well protected. Using a firewall developed by a certain AI, presented to the university free of charge by a certain ex-student.'

'n.o.body likes a creep,' said Ace.

'So instead we shall obtain the information by old-fashioned means.'

'Old ways are best,' said Raine.

The Doctor smiled. 'All we need is someone who can pick the locks on the doors and then open the no doubt equally well-locked filing cabinets.'

'It's nice to feel wanted,' said Raine.

The Doctor scrutinised the envelope on which he'd copied down the address which Raine had found as predicted in a locked filing cabinet. 'We should be there soon,' he said.

It had begun to rain and Ace switched the windshield wipers on, as they drove towards the little seaside village of Beltinge. By the time they arrived, the rain had stopped and the day was sunny once more. Everything had a freshly washed look except for the rather dilapidated house that was their destination. It was a pretty enough little cottage, but neglect had given it grimy windows and an overgrown lawn. There were three bicycles lying on their sides in the long gra.s.s, rusting slowly, and a plastic crate full of empty beer bottles stood by the front steps, as though that was as far as someone had got with recycling them.

The Doctor knocked on the door and a young woman answered. 'Ms Gulpin?' said the Doctor. 'Ms Gina Gulpin?'

'Yes?' She peered at them warily from under a fringe of hair that had been dyed dead black. Her eyes were grey and she had an unhealthy indoor pallor, oddly offset by a sprinkling of freckles on her cheeks. She was wearing indigo leggings, a black miniskirt and a navy blue sweatshirt with the words Rocketpunk Manifesto in large white letters. Long silver shapes dangled from each of her earlobes.

The Doctor smiled his most ingratiating smile. 'We want to ask you about Raymond Luthier.'

The look of wariness vanished, to be replaced by one of frank disgust. 'That rat,' she said, and turned away from the door, leaving it wide open for them to follow.

'We used to be friends, can you believe that?' said Gina Gulpin.

The four of them were sitting in the chaotic lounge which ran the length of the back of the house. A series of windows in the rear wall overlooked a small orchard of pear trees. On the outside sill of the central window stood a decorated ceramic plant pot, devoid of plants and soil but br.i.m.m.i.n.g with rainwater. The room was crowded with furniture two sofas and five armchairs, all mismatched, as well as a long wooden table with a line of computers on it. The walls were decorated with posters of lurid pulp science fiction magazine covers from the 1930s and 1940s bulbous rocket ships, bug-eyed monsters, women in pointy metal bras. There were fast-food takeaway cartons perched on every surface, some of them only recently opened judging by the intense tang of curry hanging in the air. ('Jaffrani Hyderabadi Biryani,' said the Doctor, sniffing with what might have been approval.) Only the computers looked clean and well maintained.

'We were supposed to work on our first year project together,' said Gina. 'But then he started getting all Secret Squirrel about things. Wouldn't help me with my work. Wouldn't tell me what he was working on. Next thing I knew he was knee-deep in venture capitalists and launching this AI thing.'

'You didn't want to go to the university today and hear his talk?' asked Raine.

'I'm well out of there. I was compelled to stay away by a complete lack of desire to see his smug, smiling face.'

'That and the restraining order,' said the Doctor.

Gina Gulpin glanced at him. 'All right, yeah. You have no concept of how frustrating it was. He was clever, but not that clever. But suddenly he pulls this so-called AI out of his...' She looked at the Doctor. 'His hat,' she concluded.

He smiled at her. 'If the Canterbury AI is a genuine artificial intelligence and to all appearances it is then it could be very dangerous.'

Gina shook her head, her jet-black fringe of hair swaying and her silver earrings jiggling. Seen close up, they could be identified as miniature rocket ships. 'If it is real, he didn't make it himself. He must have got someone smarter to do it. In all modesty, I'm the smartest person he knows. And I didn't do it.'

'Well, we should be able to answer these questions soon enough,' said the Doctor.

Ace looked at him. 'How are we going to do that?'

'By breaking through one of the most secure firewalls in the world and hacking into Raymond Luthier's artificial intelligence.'

'Not wishing to repeat myself, but how-'

'We use the software that our friend has designed for just that purpose.' The Doctor smiled at Gina Gulpin.

'Excellent plan,' she said, smiling back at him.

'So, this is a virtual reality system?' said Ace, looking sceptically at the two battered armchairs Gina had dragged in front of a computer on the table. Each armchair had what looked like a modified cycling helmet lying on a cushion in it, attached by cables to the computer. The computer itself was very large, and very pink, with a h.e.l.lo Kitty sticker on the top of it.

Gina nodded. 'I set myself the task of designing software that could get past just about any security measures anyone's invented yet. To make it easy and intuitive to use, I gave it a VR interface.' She grinned happily. 'And I decided not to do any of that Burning Chrome cyberpunk stuff. Instead the simulation looks like this.' She waved a hand at the pulp science fiction posters on the wall. 'Cla.s.sic s.p.a.ce battles. Planets. Starships. Alien suns.'

'And the information we're seeking,' said the Doctor. 'The data core in Raymond Luthier's AI. That will look like...'

'Three stars burning at the heart of an alien solar system.'

'Why three?'

'Because four's too many,' said Gina. 'And two isn't enough.'

The Doctor nodded. 'And the countermeasures? The software designed to stop intruders?'

Gina sighed as if it was all too obvious to explain. 'They will manifest as alien s.p.a.ceships. Armed with missiles.'

'But I'll be armed with missiles, too,' said Ace. 'Right?'

'And really powerful engines, plus lots of reserve fuel, which will give you loads of delta-v.'

The Doctor looked at Ace. 'You remember our little discussion about delta-v when you were on the good ship Vancouver?'

She grinned. 'How could I forget?'

'And what is my part in all this?' said Raine.

'You steal the stars,' said the Doctor. 'And bring them back to us.'

'Right. I see. Steal the stars.'

'Using a giant toroidal magnetic field,' said the Doctor. 'You create a jet of plasma from the star to move it.'

Perhaps bored by this explanation, Gina left the room.

'Giant toroidal magnetic field?' Raine was frowning. 'I thought someone said something about "easy and intuitive".'

'Essentially,' said the Doctor, 'it will be like a balloon with a small puncture, using the escaping gas to drive itself.'

'I have an idea,' said Raine. 'Why don't you go with Ace instead of me? You're very comfortable with toroidal fields and all that.'

'I would,' said the Doctor, picking up one of the headsets which looked like a bicycle helmet, and turning it over in his hand. 'Only these things are designed to work on a human brain.'

'Oh yes,' said Raine. 'That.'

Gina came back in from the kitchen, carrying another carton of takeaway curry and sat down on the sofa, picking up a game console with a long lead which ran back to the big pink computer on the table. The Doctor put a hand on Raine's shoulder. 'Don't worry about the details. You will just operate a simple control panel on your "starship". Subjectively it will be as easy as cracking a safe.'

'Except for one thing,' said Gina from the sofa. 'If you get dead in there you'll be dead in real life. Don't forget that.'

'Charming,' said Raine, and sank down in the armchair on the right of the pink computer. Ace was already in the one on the left, and had put on her headset. Raine did the same.

'Now you have to hold hands,' said Gina. 'Your two s.p.a.ceships will be linked together when you enter the home system and approach the Portal. Once you go through the Portal, you will be in the alien system Raymond's AI. Then if you let go, your two ships will detach and manoeuvre independently. Link up again when you're ready to come back through the Portal. I'll close it behind you to make sure none of Raymond's security software follows you back through.' She lifted up the game console. 'It could be nasty if it does.'

Ace reached across and took Raine's hand. It was warm and slick with sweat.

'This doesn't mean we're going out or anything,' said Raine.

'One last question,' said Ace, leaning back in her armchair, the headset covering her eyes and the top of her face. 'Why haven't you used this to hack in to the Canterbury AI yourself?'

'Do I look crazy?' said Gina.

Falling.

Through infinity.

The binary starship plunged on its fast, steep interplanetary orbit towards the Portal, floating in s.p.a.ce like the giant blue-green iris of an immense eye. The eye seemed to widen in recognition as it saw them coming. In its centre a disc of blackness expanded and suddenly stars were visible. Alien stars, in unknown constellations.

The binary ship which was the linked consciousness of Ace and Raine fell through the Portal. There was a shimmer, like the shimmer in your head between waking and sleeping. And suddenly they were there.

Three suns burned, a blue-white glare at the heart of a planetary system. And from those planets s.p.a.ceships were already rising in an attack formation. The binary ship split in two. Raine's craft blazed its engines and shifted into an orbit around the triad of stars.

Ace's craft continued falling on its original trajectory, until the enemy started to fire on her. And then she began a game of evasion and counter-strike. The enemy's missiles had very high acceleration, but very low delta-v. With her powerful engines and large fuel reserves, Ace was able to dodge them easily, changing orbits whenever necessary.

That wasn't the difficult part.

The difficult part was firing her own missiles, to intercept enemy strikes against Raine. Raine was too consumed with her own manoeuvres to pay any attention to defence, so Ace had to guard Raine's back, while keeping herself alive. She shifted orbit, watching a missile glide through the phantom locus of her old position, a glittering white arrow heading out of the solar system, on an eternal trajectory, never to find a target.

And she fired her own in response.

She destroyed three of the enemy s.p.a.cecraft and nineteen of their missiles at first all of these were aimed at her, but then towards the end the enemy seemed to realise what Raine was doing and concentrated its efforts on her. Ace managed to intercept every missile they fired, and she wasn't too worried.

Until they stopped firing missiles.

And something rose from the red atmosphere of a giant planet below. A s.p.a.ceship, but one unlike any of the others. Jagged, angular, black.

And big.

Far bigger than any of the others. Vastly larger than Ace's or Raine's. And moving much more quickly.

Just then Ace got the signal that Raine was finished. Ace's engine flared and she moved into a Hohmann transfer manoeuvre to match orbits with Raine. She fired her engine again, their orbits synchronised, and the two ships docked, linked, and became a binary once again.

Between them and the red planet, the jagged black ship was rising fast, a fattening spider-shape. Then the red planet turned black and the jagged ship vanished utterly into shadow. Because the light was gone. The three stars were moving, abandoning their home system. Rushing back along the same orbit Raine and Ace had followed here.

Back towards the Portal.

The stars blazed. The binary ship followed. The jagged giant pursued. It was coming after them, closing fast. Ace fired her missiles at it. Every missile she had. No point saving them now. Once they were through the Portal she wouldn't need them any more.

Every missile hit its target.

None of them made any difference.

The jagged ship kept coming.

It didn't matter. The three captive stars were approaching the Portal. It was bigger than Ace remembered. The black disc at its centre expanding to infinity. Big enough to swallow the triad of stars. They pa.s.sed through and disappeared. The binary ship followed close behind. The jagged giant was almost on them now, looming over them, dwarfing them...

But it didn't matter, because as soon as they were through the Portal they would be safe.

The Portal would close behind them.

Ace and Raine pa.s.sed through the Portal. There was a shimmer, like the shimmer in your head between sleeping and waking. And suddenly they were back.

Safe.

Home.

Now the Portal would close behind them.

The Portal didn't close.

The huge jagged black ship began to nose through, into the home system.

The Portal didn't close.

Ace reached for the controls, to fire missiles, to try and stop it. But there were no missiles left...

The jagged ship was now a quarter of the way through the Portal.

The Portal didn't close.

Something was happening on the jagged ship's fuselage. A ma.s.sive hatch was opening.