Catriona turned, found herself facing the Captain. His men, she noticed, had formed a ring around her.

'Yes?' Catriona tried to sound only irritated, tried to ignore the tension in her stomach, the thumping of her heart.

'I must ask you to come with me.'

'Why?'

'You are under arrest.'

Catriona felt a shock go right through her. She became aware of the captain's hand on his gun in its leather holster, of the other men staring at her, their fingers hooked over the triggers of their guns.

The captain reached out and took her arm, pulled her towards an Army truck parked by the side of the road. Catriona tried to pull back, but one of the soldiers caught her other arm and she was dragged towards the truck.

'What's the charge?' she shouted, beginning to struggle. 'What's the b.l.o.o.d.y charge?'

'I must ask you to come with me,' repeated the officer. Catriona wondered if perhaps that was all the English he knew.

'I said, what - is - the - charge?' she repeated, slowly. But the soldiers only dragged her onwards. Catriona saw a couple of pedestrians standing, staring. She felt like shouting for help, but knew it would be no use.

This is what 'police state' means, she thought. Jesus Christ Jesus Christ.

Someone was pulling her arms behind her back, clipping something cold and metallic around her wrists. Then they hauled her up into the back of the truck.

'You can't b.l.o.o.d.y arrest me like this!' She was shouting now, her voice echoing from the metal of the truck. She became aware that her body was shaking. 'I'm a reporter,' she shouted. 'I'm accredited by the government. Your b.l.o.o.d.y government. They can't do this.'

The captain climbed up into the truck with her. Behind him, the doors slammed. The truck pulled away, the motion throwing Catriona against the hard metal.

Slowly, her eyes adjusted to the dim light seeping in through the barred window in the door; she saw the captain, with one hand braced against the side of the truck, staring at her hard-eyed.

She tried again. 'You can't arrest me without charge. You have to tell me - '

'We don't have to tell you anything!' shouted the captain. 'You have committed treason!'

'Treason? What - '

But the captain interrupted again, leaning down so that his face was only inches from Catriona's.

'Save your breath,' he said. 'You are as good as dead already.'

Four.

Jo hesitated, glanced around the empty lab, then knocked on the door of the TARDIS. There was no reply.

She knocked harder. 'Come on, Doctor, I know you're in there.'

Silence.

He had to be in there. Didn't he?

'Doctor! Please! I need to talk to you!'

She pushed at the door; to her surprise, it swung open. The Doctor was standing at the console, his head bowed. A single yellow light flashed under his right hand. Jo ran up to him, put a hand on his arm.

'You're still worried about the Brigadier shooting us, aren't you?'

'Aren't you, Jo?' The Doctor had not responded to her touch, was not even looking at her.

Jo let go, then thought about it for a moment. 'No,' she said finally.

'I'm not. I just don't believe the Brigadier would do it.'

The Doctor turned round. Jo heard the TARDIS door shut behind her.

Jo,' he said softly. 'The Brigadier is a soldier. He obeys orders. If his commanding officer - or the Secretary-General of the United Nations - ordered him to shoot us, he wouldn't have much choice but to obey, now would he?'

Jo, stubborn, shook her head. 'He wouldn't do it, Doctor. And anyway, an order like that would be illegal. He wouldn't have to obey it.'

The Doctor began pacing up and down in front of the console. 'All right, Jo. Suppose we both became infected with an alien virus, and our continued existence threatened the lives of everyone on Earth.

Suppose the virus made us act irrationally - dangerously. What then?'

Jo bit her lip, stared around her at the white walls of the console room, the familiar yet alien roundels, the blank screen of the scanner that showed a view of nothing. She felt a cold, hard, knot form in her stomach. The Doctor sounded so sure - and if he was right - She'd been fourteen when her Aunt May had been given three months to live. She remembered her dad telling her about it, remembered running into the garden, crying with disbelief. Kicking apples on the wet gra.s.s, staring at big white clouds in a blue sky. Not believing it. Refusing to accept it.

But Aunt May had died anyway.

'What can we do, Doctor?' she whispered. 'There must be something we can do.'

The Doctor walked up to her, took her hands, gave her his most rea.s.suring smile. 'Well, the first thing we have to do is split up.

Whatever is going to happen to us, it will almost certainly happen when we're together. So as long as we stay apart, we're fairly safe.'

Jo thought about this for a moment, frowned.

'But surely we're safe as long as we stay out of the laboratory?

That's where it happened - where it's going to happen, I mean.' She felt her stomach lurch as the meaning of the 'it' she was talking about came home to her again.

'Not necessarily, Jo. Have you ever heard of a bell distribution?'

Jo frowned. 'It's something to do with statistics, isn't it?'

The Doctor smiled. 'That's right. Well, what the Prognosticator shows is the middle part of the distribution - the most probable sequence of actions, if you like. Around that are a lot of less probable sequences it doesn't show - '

'Like where the Brigadier misses us, or doesn't shoot us at all?'

asked Jo excitedly.

But the Doctor shook his head. 'No, Jo. Like where he shoots us in the car park, or in the radio room. Or where he uses a different gun, or it happens a day later or a day earlier. The probabilities you're talking about - where the key event is different - are very small ones indeed. That's why it's more than ninety-nine per cent certain that something very like what we saw will happen.'

'But we can make it less likely?' Jo couldn't quite squash the feeling of hope growing inside her.

'It should be possible, in theory,' said the Doctor. 'The trouble is, I don't know how at the moment. If I'd managed to get a fix before the tri-capacitance circuit shorted we'd have known more, but as it is the best thing you and I can do is to keep away from each other as far as possible.' He walked over to the lockers, opened a door and took out a device the size and shape of a transistor radio, with an odd pattern of coloured b.u.t.tons on its surface. He pressed one of the b.u.t.tons with his thumb, said, 'Say something, Jo.'

'Anything?' she said doubtfully.

The Doctor smiled again. 'Yes, "anything" will do nicely.' He pushed another couple of b.u.t.tons and handed the device to her. 'Now that I've set it up, this device will only respond to your voice, Jo.

What you should do is find out what's happening around the HQ -

anything strange, anything at all - and then record a message for me, telling me about it. You need to press the blue b.u.t.ton - this one - to record. I suggest that after -' he glanced at his watch '- two hours you leave the device on the lab bench, where I will pick it up.

Then leave the area of the lab. After another hour, come back to the lab and pick up any instructions I leave you - you'll have to say "recall" into the device, and press the yellow b.u.t.ton. Is that clear?'

Jo looked at her own watch. It was three o'clock.

'Five o'clock. "Recall". Yellow b.u.t.ton. Okay, Doctor.'

The Doctor half-turned to the console, then turned back to face her.

'Oh, and Jo - '

'Yes?'

'Good luck.'

He extended his hands, and Jo rushed forward, hugged him, her head against his chest. She knew that they might not meet again, or that if they did they might only have a few minutes left to live. She wanted to say a lot of things. She wanted to say that being his a.s.sistant was better than being a spy. She wanted to say that he was like a second father to her. She wanted to say that he had shown her the wonders of the Universe, and that there weren't the words to tell how she felt. But she didn't say anything much in the end, only a m.u.f.fled, 'Goodbye, Doctor.'

Then, quickly, before she could panic or change her mind, she stepped out of the TARDIS and into the lab. It felt strange, somehow, almost as if it were another alien planet. As the door shut behind her, she saw the pile of photos and the guide to Kebiria that Mike Yates had given her, lying on the lab bench where she'd left them. She turned back to the TARDIS.

'Doctor - '

But the whistling, roaring sound of dematerialization had begun.

'Doctor!'

The TARDIS faded from view. Jo looked around her, looked over at the doorway. She remembered the image of her own body slumped by the door, remembered the blood staining her T-shirt.

She pressed the blue b.u.t.ton on the recording device.

'Well, Doctor,' she said. 'At four-oh-two the TARDIS dematerialized, with you in it.' She paused, looked at the doorway again. 'I hope you're coming back,' she said.

Brigadier Alistair Lethbridge-Stewart thought about killing people, and decided that he didn't like it very much.

He thought about killing the Doctor and Jo, and shook his head.

'Impossible,' he muttered. 'Quite impossible.'

But on the other hand - He tried not to think of the circ.u.mstances in which he would have to shoot them. He knew there were such circ.u.mstances. They were conceivable.

'But the Doctor's wrong,' he muttered. He imagined the Doctor standing in front of him, immaculate in his peculiar costume of velvet and lace. 'No. This time, Doctor, you're wrong.'

There was a knock at the door.

The Brigadier looked down at the paperwork he was supposed to be doing, sighed. 'Come in.'

The door opened and Captain Yates stepped in, saluted casually. 'I need your approval for an ENA team, sir,' he said without preamble.

'We need to look at the Gilf Hatar anomaly.'

The Brigadier raised his eyebrows. 'The what what anomaly?' anomaly?'

'Kebiria, sir.'

'Kebiria? Isn't that Captain Deveraux's patch?'

Yates looked down at the carpet.

'We've lost Deveraux, sir. He's been missing for a couple of days and -' He told the Brigadier about Catriona Talliser's telephone call.

Lethbridge-Stewart felt his heart sink. Another one gone.

'This - reporter person,' he said, when Yates had finished. 'Is she sure about the ID? I mean, have we got any corroboration for this?'

Yates nodded.