'What is it?' she said.

'Stage-two sterilisation,' The Doctor told her. 'Come on.'

They backed off with haste. Elsewhere in the liner Tegan and Turlough were yelling in an attempt to get their attention, but it was too late. The heavy gas deadened any s.p.a.ce that it filled, and now it seemed to be coming from every direction. With no handy vents and no alternative air supply, the Doctor knew that their chances of riding out the sterilisation were, as the automated voice had put it, small.

They were more than half-way to the exit, as the Doctor remembered it. Not an attractive course to take but then they didn't have many options to choose from.

The door to the outside was dropping as they reached it, eyes streaming and gasping for breath. Kali would have done better if she'd kept a hold on her pressure helmet, but both she and Olvir had left them in the control room. They were a liability in combat, and they'd seemed unlikely to be necessary for a trip in the TARDIS.

They ducked under the falling edge of the door and emerged onto the receiving platform. Kari was already ahead, her burner raised and at the ready.

'I'm used to this,' she said, suddenly business-like and unarguably in command. 'Stay with me.'

The Doctor wasn't going to object. Kari had been trained in making sudden entries to strange and probably hostile situations, and such an advantage wasn't to be wasted. He said, 'What do we do?'

'First, we get to cover.'

No disputes so far. The receiving platform was as brightly lit as a boxing ring. The elevator shaft was empty and there was only one way to go, down the iron stairs to the side.

Even as they moved, the lights went out.

The Doctor was going to wait until his eyes adjusted, but Kari had a hold on his elbow and was pulling him along. He groped blindly for the guiderail, found it, and began to follow her down. They took it slowly, being careful to make as little noise as they could.

Within a minute, he could see. There was a dim glow around them, no more than a starlight overspill from the brighter areas somewhere down below, but it was enough. They were on part of a complex of catwalks that centred on the elevator shaft. Some ran along girders bolted between uprights, others were cable-suspended over long drops through darkness.

Where two walks crossed over, a ladder or stairway would connect them. The entire structure appeared makeshift and frail.

Kari studied the way ahead. She was aware of the lit areas down below, and she wanted to pick a route which would avoid them. The object was not to seek confrontation, but to find somewhere away from danger so that they could discuss and decide their next move.

As she was evaluating, the Doctor was marvelling.

He'd moved to the catwalk rail and was looking down on the same scene that had appeared to Nyssa: the vast interior of the Terminus, and the antlike activity under the bright lights in a small section of it.

'Dante would have loved this,' he breathed a living h.e.l.l, complete with armoured dark angels.

'Reconnaissance comes later,' Kari said, and she pulled him away.

From his place by the lighting switches three levels below, Valgard watched them go in amazement.

Outsiders? In the Terminus Terminus?

The area that Kari found for them seemed to be some kind of storeyard. It was on the 'ground-floor'

level of the Terminus, but it was away from the occupied areas and further screened by a number of hung tarpaulins over a frame of scaffolding.

'The liner's no good to us now,' Kari said decisively.

'We'll have to find another way out.'

'You're combat section,' the Doctor reminded her.

'Leave the strategy to me.'

'But what's the alternative?'

'We've got Olvir and Nyssa to think about. Nyssa may be hurt you saw the blood on the floor. I've got friends back in the TARDIS and they're trapped as surely as we are.'

'But we can't go back,' Kari pointed out.

'No,' the Doctor agreed, 'We can't. But in the end, we may have found that we had to come out into the Terminus anyway.'

'But why?'

'There's not only escape to think about. We take the risk of Lazar infection with us. And if there's an answer for that, I think we've a chance of finding it here.'

The Doctor pulled back a canvas cover. Underneath it was a stack of highly polished metal sheets standing on end. He looked at the distorted reflection of his own face. Nothing of the Lazar disease showing there... but for how long?

Kari said, 'You think there's a cure for the disease?'

For a moment, the Doctor said nothing. He moved on through the storeyard. Finally he said, 'I think there's more to the Terminus than just an old dead ship.' Now he stopped before some kind of signal box that had been bolted to an upright. 'Didn't your chief think that there was anything strange about its position on the charts?'

Kari didn't answer. The Doctor let her chew on the idea for a while before he turned for her reactions.

Kari hadn't spoken, not because she was lost for a response but because a metal staff clamped crosswise on her neck was cutting off her air. Valgard had managed the hold in such a way that she could neither cry out nor reach her burner. Almost as the Doctor saw them, he released her. She slid to the floor in a graceless heap.

And then Valgard came for the Doctor.

The armoured Fury with its mailed hands outstretched, no part of the human being visible, would have been enough in itself to overcome opposition in many, and even the Doctor, who had seen more than his share of strange sights and weird aggressors, hesitated for a moment before he could react.

It was long enough. Valgard's hands clamped around his throat and started to squeeze.

Until now the Doctor hadn't been certain as to whether Valgard was a man or an artefact, but the pressure behind the gloved fingers was human. It was a limited kind of relief hydraulically powered pincers would have decapitated him as easily as one might snip the head off a flower. The Doctor grabbed at Valgard's arms and tried to relieve the pressure, but Valgard responded by bearing down more heavily.

They struggled in silence. The Doctor wasn't having much success. Everything started to turn grey, and then red; and as blackness started to creep in from the edges of his vision, the Doctor knew that the situation was becoming desperate.

He could see, dimly and far away, that Kari was stirring. Her speed of reaction was a tribute to her training. Within a few seconds she was fully alert and reaching for her burner.

Some sign of hope must have shown in the Doctor's eyes. Valgard swung him around. The pressure eased for a moment, and then the Doctor was shielding the Vanir from Kari's weapon. There was no way that she could get a clear shot.

She fired.

The burner spat a continuous red beam. She'd opened it up to full intensity. She was aiming wide of the mark, and the Doctor could immediately see what her intentions were. Valgard couldn't... but then, that was the idea.

Kari was aiming at the reflective sheet that the Doctor had uncovered only a couple of minutes before. A couple of minutes? It seemed like hours...

but then the Doctor realised that he was losing his hold on consciousness, and he fought to get his mind back in focus. The energy beam was being reflected from the sheet at an angle which took it only a metre or so behind Valgard's all-enclosing helmet.

The less-than-perfect reflectivity of the surface meant that the beam was starting to get diffuse as it came close, but it would have to do. The Doctor pretended to weaken suddenly, and Valgard was so taken by surprise that he almost overbalanced. He was even more surprised when his victim came surging back with renewed strength, enough to force him back a pace. And then another.

Valgard's helmet pa.s.sed directly through the path of the beam. There was a searing flash and a sound like lightning in water, and suddenly it was all over.

Valgard clutched at his head and fell with a crash.

The Doctor felt as if he'd been the tester in a noose-tying contest. Any more, and he was sure that he'd have been carrying his own head around in a bowling-ball bag. Valgard was making weak struggling motions, trying to get his helmet off. He was down, but he certainly wasn't out.

Kari came over and stood by the Doctor. She took the back-up power pack from her belt and plugged it into the burner. That one long burst of energy had drained it completely. She said, 'Is it a machine?'

'It's a man.' Speaking was like spitting gla.s.s, but it didn't feel as if there had been any permanent damage. The Doctor went on, 'He's wearing radiation armour. Keep him covered.'

Valgard was already making the effort to sit up.

Kari said, 'There's a problem.' She said it in the quiet, unexcited way that people save for the worst disasters.

'What do you mean?'

'The back-up unit's dead. I've no power.'

Valgard had made it to his knees, and they had no way of stopping him.

'Come on then,' the Doctor said. When it came to a choice between fighting and running, the Doctor preferred to run every time. Those who stayed to fight tended to be swiftly stripped of their n.o.ble illusions.

They took aim for the darkness, and ran.

Valgard struggled a little longer, and finally managed to remove his helmet. It had protected him from the worst of the blast, but the heat had sealed all of its ventilation lines and crazed the one-way gla.s.s of the visor. He'd been blind and baking inside the useless piece of armour was trailing steam as he let it fall to the floor. Flushed and panting, he looked around. The intruders were gone, but the sound of their running footsteps echoed back to him.

He'd followed as far as he could, and he could follow no further. They'd gone straight into the zone.

'Anybody coming after us?' the Doctor said when they stopped for breath.

Kari checked behind them. No.'

'Let me have your radio.'

She handed it over without question. Now, more than ever, they needed to get a warning to the others wherever they were. But she'd misunderstood the Doctor's intention. She kept watch for pursuers, saying, 'If they wear radiation armour, there must be radiation.'

'That's what I'm checking,' the Doctor said, and he held the radio out at arm's length and switched it on.

A pulsating waveform came through, strong and loud. It was similar to the interference they'd first heard on the liner, but it implied a much more serious leak. Kari said, 'Badly shielded engines again. Always the same pattern.'

The Doctor switched off the radio. They could forget about using it to communicate. There were properties of interference here that he'd never encountered before. He said, 'What kind of engines are they?'

'A self-containment reaction drive. It's like building a big bomb and then using the blast energy to form a container. Then you can skim off power whenever you need it.'

'No need of fuel, and it runs forever. What happens if anybody plugs the leaks?'

'You don't wait around to find out.'

The Doctor handed the set back to her. 'Let's move, then,' he said, and started out. Mari hesitated momentarily before she followed. She'd always believed that she could sense when she was being observed, and it had saved her in a couple of tight situations in the past. Now it seemed to be playing her false; there was a definite tingle, even though the more she looked the more certain she was that they were alone in the depths of the Terminus.

She put it out of her mind. That dull red gleam could have been anything.

The tank that Nyssa had come to think of as the Lazars' ward was bare, not too clean, and very poorly lit. Nyssa, like most of the others, sat on the floor by one of the walls. The worst cases were lying at the far end of the tank, in rough bunks, stacked like shelves from floor to ceiling.

She tried to use the time to do some coherent thinking about her position and the courses of action that were open to her, but concentration wouldn't come. It was like trying to catch hold of a spot of light on a wall.

So when two of the Vanir entered the tank and began checking the Lazars one by one, Nyssa was starting to get desperate. They'd left their helmets by the door (why did they seem so unafraid of infection?) and she recognised one of them from the receiving platform. When they got near enough, she'd speak to them.

This was Sigurd's least favourite part of the whole operation, lifting heads and looking into one pair of dead eyes after another. As they moved along the lines he reported symptoms and made estimates on the chances of each Lazar making it as far as the zone.

Some of them wouldn't even leave the tank alive. The other Vanir dutifully noted everything on a clipboard.

'I want to speak to somebody in charge,' one of them said suddenly as they came level. If Sigurd recognised Nyssa, he didn't show it.

'Speech centres untouched,' he dictated, 'could be a remissive.' The other Vanir made a note.

'Please listen,' Nyssa said, and reached out for his arm.

Sigurd caught her hand and tested its flexibility.

'General weakness,' he said, 'poor grip. But make a special note for Eirak.'

He straightened up, and the two Vanir moved on.

Nyssa sank back, weak and defeated.

'You'll get nothing out of them,' the Lazar next to her whispered. 'They're not interested.'

Nyssa looked around in surprise. She'd come to believe that none of the Lazars was capable of speech, but the one alongside her was lifting back with difficulty the cloth that covered its head. This revealed a girl, a pale blonde of about Nyssa's age. She wasn't as far gone as any of the others, but the disease was surely squeezing the life and strength out of her.