'You arrived on this planet a very long time ago. Your job was to report on the research that was being carried out here. You overstayed your welcome and contracted the disease. Desperate to survive, you abused your position.

You made sure that you would survive, even if everyone else died. The surgeons literally hacked out the infected tissue and stuck in cybernetic parts. All your sytems, including your brain, were given electronic backups in case of further degeneration. Then you tried to flee in a shuttle craft, taking with you a single Mecrim specimen in a cryogenic casket. For some reason there was an explosion.

The cybernetic and computer systems ensured your survival, but only by causing a shut-down that lasted for centuries. Over time the automatic systems replaced damaged parts and rid you of every hint of infection. You were a guinea-pig in an advanced self-replicating cybernetic experiment, and you outlived the scientists who operated on you by hundreds of years.'

'This is ridiculous!'

'There was, however, a problem,' continued the Doctor with scarcely a breath. 'I can only guess that the software to control the brain was almost entirely wiped by the explosion. The microprocessors were built to learn, to adapt - but the only thing they remembered was that technology and science were, in some indefinable way, evil evil.

'You awoke in mental darkness and confusion, and sought meaning and ident.i.ty in the nearest inhabited place. You insinuated yourself into society, and took on the role that most suited you - that of a Knight of Kuabris. You doubtless learnt quickly, rising through the ranks, but never speaking of your past - because you haven't got one.

'I can only guess what happened next, but no doubt you found yourself being drawn to those few Kuabris texts that remained from the previous Grand Knight. Before Himesor was made Grand Knight you had already stolen one of the holy robes. Some semi-dormant computerized memory told you that the suit would offer protection against the deadly gases of the sewer. Over a period of time you - and others

- discovered various entrances to this submerged city that avoided the sewers. The irony is you needn't have bothered.'

The Doctor paused, aware for a moment of the sounds of the Mecrim fighting each other. For the first time he could see the lights of the power station somewhere in the distance.

'You see, Zaitabor,' said the Doctor, staring down at the unit in his palm, 'you aren't human.'

Nineteen.

Your words are meaningless,' said Zaitabor, 'You speak the unintelligible sounds of science and expect me to -'

The Doctor stared at the small image of the man in the palm of his hand. 'Really? Are you sure that you do not understand me? Tell me, what are your earliest memories?'

There was a faraway look in Zaitabor's eyes, his words stumbling. 'Nothingness . . . Then the ability to grapple with the void, giving it definition, knowing that by being able to a.s.sign a word to my condition I must be alive .. . Then a rush of input, raw data, so much I could not put words to.

Darkness again, and then a gradual letting-in of information.

Surroundings, distinct from me, distinct from the receding darkness at the core of my being. Realizing that there were things beyond me, that I was a unit in a sea of data . . .

Seeking definition from surroundings, from other units.

Understanding life.' Zaitabor's face hardened in an instant. 'I remember nothing of my childhood. My first memory is offering myself to the Higher and the wisdom of the way of Kuabris.'

'You had no childhood!' said the Doctor. 'At least, not in this life.'

'I am a man!'

'You can fool yourself, but you cannot fool your own augmented biology. Have you ever been ill? Have you ever felt tired? How much of your humanity is a clever, randomized program, with hungers, desires and dreams put on for show, disguising the emptiness of your soul?'

'This is a mere distraction,' said Zaitabor, turning away.

'You are trying to confuse me, to thwart the Higher's great plans.'

'I've seen the image of the man you once were. You look a little better for the weight you've lost, but you have the same diseased mind.'

'I shall initiate the final sequence,' said Zaitabor. 'The core will overload.'

'Where did you learn such words?'

'I am not justifying anything to you!' screamed Zaitabor.

'This dark city will be destroyed!'

The Doctor sighed. 'I had hoped to reason with you, but it seems that you find this particular human concept alien.

Still, one way or another, I shall stop you,' he affirmed.

'Any attempt to stop me will result in the boy's death,'

said Zaitabor, pulling Cosmae back into the picture.

'Who will die when that reactor goes up anyway,' said the Doctor.

Zaitabor pressed the knife hard into the young man's throat. 'You want to see him die now?'

'No,' said the Doctor. 'I promise that the android will make no moves against you.'

'And yourself?'

'I never make promises I can't keep,' said the Doctor, flicking off the comm unit. He turned to Jamie, Zoe and the others. 'I've got one chance to get rid of the remaining Mecrim,' he said, 'and stop the power station exploding. But I'll need something that can get me to the power station fast.'

'I've seen lots of old fliers and hover vehicles around the city,' said Zoe. 'I'll try to restart one of those.'

'Good,' said the Doctor. 'Do you think you could bring one back in, say, five minutes? Take Jamie with you.' He pointed towards the far end of the park. 'I'd suggest going in that direction. It looks like the Mecrim will stay here for a while.'

'We'll be back as soon as we can,' said Jamie, following Zoe.

'And what are you going to do?' asked Defrabax.

The Doctor glanced in the direction of the Mecrim. There were about fifteen of the creatures still standing, and they tore into each other with unquenched aggression. Heads were crudely hacked off, legs snapped, rib-cages shattered.

The edge of the park was a grisly blur of movement, watched by shocked Dugraqs and hovering Taculbain.

'The Mecrim are attracted to large animals, as the twins found out,' he said, nodding in their direction. He pointed to his gun. 'With this we were able to fool a number of Mecrim into attacking their own kind. Now, I'm going to reprogram the entire gun and the last cartridges it contains. I want it to give off the heat signature of a boiling blue whale. And then I'm going to lead the last Mecrim to their doom.'

'We have a legend of such a man, who mesmerized verminous beasts and led them to the sea,' said Defrabax.

He caught the Doctor's eye. 'But now is probably not the time to recount it,' he added hurriedly.

Zoe and Jamie ran across the muddy plain that was once the park's playing fields. 'There's one,' said Zoe, pointing to a flat area of concrete. It still bore the faint lines of parking s.p.a.ces, although in the semi-darkness they resembled strange geometric shapes unearthed by an archeological dig.

A single metallic-blue object sat in one of the bays.

'And you're expecting to get one going after all this time?'

'If they were well designed, there's no reason why not,'

said Zoe. 'Whatever geological or other processes covered this city seem to have preserved its contents reasonably well.'

Zoe ran up to the machine and within moments had lifted the covers from the engine housing.

'Where are the wheels?' asked Jamie.

'There aren't any,' said Zoe. 'It floats over the ground.'

'Oh,' said Jamie. 'I see.' He ran his hands over the windscreen and the faded seats, then opened a door to look at the c.o.c.kpit controls. 'Do you want me to press anything?'

he asked hopefully, eyeing the switches and dials.

'Not yet,' mumbled Zoe, holding a number of rubber-coated wires in her teeth.

'Well, I'll get in, just in case,' said Jamie, slipping in behind the steering wheel. 'I've always fancied driving one of these things.'

'One of what things?'

'One of these. Whatever it is. Does it work like the jeeps we saw when we battled the Cybermen?'

'This is much more sophisticated,' said Zoe proudly, replacing the engine coverings. 'Quite long-lasting, too.' She jumped into the seat next to Jamie's. 'Not bad for a soft-top family model.'

'Aye,' said Jamie knowingly. 'Not bad.'

'I've had to by-pa.s.s all non-essential circuitry - lights, electric windows, that sort of thing.'

'Right.'

'And there's about ten minutes' power left in the emergency cells.'

'Ten minutes. That's good. Now, which b.u.t.ton do I press?'

'That one.'

The vehicle began to lift off from the ground, Jamie grinning brightly the whole time.

'Now,' said Zoe. 'Press that to disengage the parking lock, and this to go forwards.'

'You sure?'

'I think so,' said Zoe. 'I've never actually seen this model before.'

The hover vehicle shunted backwards slightly, b.u.mping into something. Zoe turned to look behind. 'I don't remember there being a wall -'

A Mecrim stood behind them, its claws already deep into the plastileather back seats.

Zoe screamed and Jamie thumped the controls until the craft shot forwards. The rear end sc.r.a.ped along the rushing ground as the Mecrim struggled to climb in.

'Faster,' said Zoe. 'Faster!'

'I'm trying,' said Jamie.

The hover vehicle shot over the park like a dart through the air. Jamie was thankful that he hadn't yet been called upon to steer or stop. He pressed another switch, hoping to increase speed still further, and succeeded only in causing the left side to tilt towards the ground.

'Turn that off!' snapped Zoe.

The Mecrim's long arms were reaching out towards them, scrabbling at the back of their seats. Jamie felt a claw rake down his back and bit his tongue against the sudden stinging pain, edging forwards in his seat. 'There's a gun in my belt,' he said, swerving the flier around a tree.

Zoe grabbed at the revolver and turned in her seat, her hands shaking with unfamiliarity and fear. She pointed the business end in the rough direction of the creature and pulled the trigger.

There was a loud crack. She estimated that she'd missed the Mecrim by several metres.

She tried again, gritting her teeth and narrowing her eyes.

This time she had the satisfaction of seeing the bullet thud into the beast's chest, but it seemed to make no difference.

'Hang on,' said Jamie. 'Let's try something else.' He began to rotate the wheel from side to side. A couple of times the pitching vehicle hit the dried mud beneath them, gouging furrows in the earth and threatening to topple over.

Still the creature clung on, its long legs seeking to push it further into the machine.

Jamie scanned the controls in front of him, trying to make sense of the pictograms and symbols. 'This looks interesting,' he said, pushing another switch.

There was a whining sound from the rear of the vehicle, metal arms and clear plastic rising into position. As the roof tried to swing upwards it pushed at the Mecrim, which started to lock its primitive arms around the frame.

Jamie pulled down hard on the steering wheel and the roof mechanism shrieked. There was a thump and the Mecrim was gone, tumbling along the ground.