The computer systems were up and running and the Doctor began to tap doubtfully at a small keyboard. 'No, that's not right. I wanted . . . Ah, here we are.' The top half of the screen was split into two alternating real-time video camera sequences, showing the state of the reactor. Even the Dugraq could tell that things were in a pretty bad way. The bottom half presented a number of simple graphs. Each had a large splash of red encroaching over the screen. 'Oh my giddy aunt,' said the Doctor. 'We've got less time than I thought.'

'I can only contact Defrabax in an emergency,' said the implacable android.

'Well, I think this counts as an emergency, don't you?'

said the Doctor.

Zoe and the twins edged around the walls towards the still-open door of the furnace. The creature was tracking them from a distance, but for the moment had not come any closer. Perhaps it had been injured by Diseaeda's attack - or by the station worker's torch - but Zoe felt that their luck could not last much longer.

Just as they were almost within reach of the huge metal shutters the creature sprang into the air. The twins threw themselves to one side but a fist of razor-sharp talons clamped around Zoe's thigh and twisted her savagely to the ground. As her head thumped against the floor she saw sparks and splashes of colour, and then the monster raising a claw into the air, preparing to smash it downwards.

She heard the awful sound of muscle and bone rupturing, but it was not her own. She looked up, her vision still blurred, and saw the unmistakable form of Diseaeda.

He had recovered consciousness and was brandishing one of the metal bars. He was very pale, seeming to have lost a lot of blood from somewhere, but his sudden strength was like that of ten men. He took a few steps away from the creature and then gripped the bar firmly, angling it forwards like a spear. Then he charged forwards.

Whatever the intention of his attack the results were clear enough: the huge bar sank into the creature's neck, just under its skull.

The beast turned, the bar clanging into the wall and then falling to the ground. Zoe noticed a deep wound in the creature's neck, a round slot big enough to accommodate a fist. Blood gushed over its shoulders and down its spine.

The animal was now standing a few feet away from Zoe.

Pressing her hands tight over her leg wound and gritting her teeth against the pain she began crawling along the base of the wall. Even through the agony of her wound she could see the circus master standing erect in front of the creature.

The twins stood at his side, shaking with fear. They had run to the worker's corpse to pick up the welding torch, but Raitak held it uneasily in her hand. The twins did not know what to do.

Diseaeda did. 'I've had enough of this,' he said, and reached for the torch. 'I'm much too old for this sort of thing.'

With that, he twisted the controls of the device and casually dropped it to the floor. A wide fan of flame shot upwards like a screen between him and the monster. Zoe could barely see through the burning, shimmering air, and presumed that the creature could not either. But why had Diseaeda discarded perhaps the only weapon to have had an effect on the beast?

The creature could no longer see Diseaeda and the twins through the haze. It half-turned, searching for Zoe.

In that instant Diseaeda jumped through the wall of fire and hurled himself at the creature with every ounce of his remaining strength. His hair was burning, his clothing smouldering, but his eyes were glazed over with a single purpose. His large body thudded into the creature, catching it off-balance. In Zoe's mind it seemed that the two humanoid figures hung in the air for a moment, two opposing forces smashed together. She saw the creature's blank face, half-twisted away as if in disgust for being surprised by the same man once more. She saw Diseaeda, the flames licking away his age to reveal a young man, and then a frightened boy.

They arced through the air, merging long before the flames of the furnace reached out to them, four arms wrapped tightly around the man but unable to prevent the momentum carrying them backwards. The creature's head and shoulders, already damaged by fire, crumpled first, and then the red spears of heat stabbed into the circus master's chest and legs.

There was a small, glimmering explosion, deep in the inferno of the furnace.

The Doctor followed the android as it walked unhurriedly into the main room and stood before the large screen, tapping a few controls in the arm of one of the chairs on the dais.

The screen glowed for a moment with phantom images, and then showed a solid wall of static. The android removed the clay pendant from around its neck and chipped away at it with its ma.s.sive hands. A few moments later a small silver wand was revealed, which it placed into a slot by the screen.

The image stabilized to reveal the logo of an ancient piece of communications software. A few seconds later the screen showed a balding grey-haired man, a damp felt hat pulled down over his alert but disgruntled eyes. The image swayed unsteadily, and the man seemed to peer down at the Doctor and the others. Clearly Defrabax - if that was he - was holding some sort of small communications unit in his hand. 'What is it? You should have powered-up by now.

And what are those people doing -'

'There is an emergency, master,' said the android.

'This is what is called a secure channel,' snapped the magician. 'And I'm halfway up a very lonely hillside, watching the city. You can stop pretending to be my homunculus for a moment. No one's going to hear you.'

'The reactor is dangerously unstable, Defrabax. It must be closed down. Even at its present output it will explode in less than five minutes, causing an explosion equivalent to 790,000 kilos of plastic AK.'

'But you said that the figures were acceptable.'

'The figures accessible from the main control room indicated that the reactor was just running within the manufacturer's parameters. The test room shows that the reactor core is running over 25 per cent above recommended tolerances.'

'But how did you get into the core testing room? I thought you said that there was only one key, the key that -'

'This man opened the door for me.'

The Doctor stepped forward to the android's side and doffed an imaginary hat. 'Good evening - or is it morning up there? It's so difficult to tell. It's very dark down here, you see. I'm the Doctor, by the way, and if we don't shut this thing down soon I shall never get to meet you. It's Defrabax, isn't it?'

The old man nodded. 'That's right. And what business do you have down there?'

'The knights forced me to join an expedition in search of the Menagerie of Ukkazaal.'

'Then you're responsible for this?'

'No,' said the Doctor. 'I'm no friend of the knights. I'm here purely to avert a catastrophe.'

'I don't believe you. I'm not going to stop now. Not after these long months of preparation.'

'Ask your android,' said the Doctor. 'You know he'll speak the truth.'

'The Doctor is correct in what he says,' stated the android.

Defrabax appeared to be on the verge of tears. 'But I'm so close to success . . .'

'What are you trying to do?' asked the Doctor gently.

'I want to use that technology for the good of all our people.'

'And do you understand every aspect of what you believe to be down here? Do you have any idea of the centuries of learning that gave rise to this little microcosm of human civilization?'

'Of course not,' said Defrabax. 'The android has outlined certain principles. I have found it difficult to work only from the surface, to not actually be able to go down to the old city and see things for myself.' The man sighed. 'And, in any case, how much could I learn? I am but one man. A man of strong ideas but, it seems, little ability to back them up.'

'Now, I wouldn't say that,' said the Doctor. 'But the point is -'

'Where does your knowledge come from?' asked the mage. 'I can't believe that there is any region on our world as advanced as the old city.'

'I am from far beyond your world,' said the Doctor. 'But let's talk about such matters later. The important thing is that we shut down the reactor now.'

Defrabax sniffed. 'Very well. Android?'

'Sir?'

'I see that the Dugraqs have discovered my . . .

selfishness. Perhaps I have abused the trust of those that gave you to me. Change your command programming to obey the Doctor. Do everything he says.'

'Very well,' said the android, turning to the Doctor for instructions.

'I want you to know that I am not unsympathetic to your aims,' the Doctor told Defrabax. 'Once we've closed this plant down we'll meet you on the surface.'

'Very well,' said Defrabax. He reached down to switch off the device and the large screen again filled with the white and blue logo.

The Doctor smiled at the towering form of the disguised android. 'Now, I want you to undo everything Defrabax has instructed you to do. Shut this place down, obeying all safety protocols. How long will that take?'

'Approximately fifteen minutes to shut down major systems. Peripheral work will take a further six to seven hours and -'

'That can wait. Just make this place safe.'

The android nodded stiffly and moved off.

The Dugraq turned to the Doctor. 'Defrabax's heart is full of good intentions.'

'I'm sure of that,' said the Doctor. 'But he needs to realize that it takes time to develop technology like this. His society isn't ready for this level of sophistication - in fact, few ever are. In the world over our heads gunpowder is unknown. The people go as far as their legs or horses can carry them. They have largely managed to avoid polluting the rivers or destroying entire ecosystems by intensive logging. It would be a brave man to say that they're more primitive than their ancestors.'

'What do we do now?' asked the scout.

'I will ensure that no one else can operate this machinery.

And then we'll go back up to the surface, preferably avoiding the sewers this time. I need to find Jamie and Zoe, and I want to find out who arranged the Taculbain ambush.'

The Doctor smiled sadly. 'A number of people have died already. I don't propose that there should be any more before I leave this planet.'

Zoe stood next to the twins. 'I'm very sorry,' she said.

'He shouldn't have done that,' said Raitak.

Reisaz nodded. 'He's saved our lives twice now. Once should have been enough.'

Zoe pulled a makeshift tourniquet tighter around her leg.

The bleeding seemed to have stopped now. She would need a few st.i.tches but other than that she'd be all right. But there was no sense of victory in her mind. Diseaeda had sacrificed himself in order to destroy a lethal and uncontrollable creature let loose into a civilization that had no real defence against it.

They closed the metal doors over the furnace and then slumped down to the floor, too exhausted and shocked to speak. After a period of contemplation the twins found a herbal lotion in one of their pockets which they dabbed around Zoe's wound and on their own blisters.

And then the twins started crying, great whooping, choking screams of uninhibited grief.

Zoe stared ahead, stoically.

Some minutes later Raitak blew her nose on her sleeve.

'It'll be dawn soon,' she said through sobs.

'What will you do?' asked Reisaz, rubbing her eyes as she looked at Zoe. 'Under our law you're free to go now, of course.'

'I'll go back to the city, find the Doctor and Jamie,' replied Zoe. 'When I tell the Doctor what's happened I'm sure he'll make sure that the other creatures stay in suspended animation.'

'We'll come with you,' said Raitak. 'The circus can wait.'

'You'll be going back to the freak show?' asked an incredulous Zoe.

'On Diseaeda's death not only are all his servants and helpers liberated,' said Raitak, 'but we become its new owners.'

'It will be good to see you safely reunited with your friends,' said Reisaz.

'But most importantly of all,' said Raitak. 'We must ensure that there are no more creatures like that thing we have just destroyed.'

Fourteen.

The silver globe of the sun rose slowly through the early morning mist. Woods and farmhouses were rendered as dim grey shapes in the distance. Only the road ahead was straight and true.

Zoe turned to look at the twins. They had returned to the circus to retrieve Diseaeda's favourite horse and to find a suitable creature for Zoe to ride. She was grateful for the comparative comfort of her return journey, but was shocked by the grey faces of the circus workers and townsfolk as they cleared away the bodies. The twins had been numbed by Diseaeda's death, and conversation had been difficult.

Zoe had seen many deaths since beginning her travels with the Doctor and, although her faith in him was absolute, that never meant that coping was any easier. And her grief was always second-hand, like sobbing at a motiveless murder reported by the telepress. What were those affected actually going through?