'Can I hold out?' the Doctor asked. He smiled, a genuine open smile rather than the manic possessed grin of the Denarian. 'My metabolism isn't bothered by that sort of genetic nonsense. There's more than enough going on inside here already.' He patted his chest. 'That sort of alien material would be neutralised and rejected immediately.'

'You're immune?'

'Evidently.' The Doctor sat himself down on the stool next to Sheldon. 'But that doesn't surprise me. In fact, I was rather counting on it. Unfortunately I don't think my immune system is straightforward enough to duplicate and transplant into the entire population of these benighted isles. Even if I thought that was a good thing to do, we'd only be swapping one alien metabolism for another.'

Sheldon just stared at him.

The Doctor waved a hand dismissively in the air. 'Doesn't matter.' He hunched forwards, hands clasped together on the workbench. 'What I'm interested in is why you are suddenly so much better. Since you received a second dose, you seem rather more coherent, if you don't mind me saying. Rather more what I imagine to be yourself.' He raised a quizzical eyebrow. 'Now that may be something we can duplicate. Do you think?'

'Twenty-seven oh six.'

'Are you sure?' Janet asked.

Sir Anthony Kelso nodded. 'His late wife's birthday.

Packwood used it as his code for everything. Even as the pa.s.sword for his secure systems. Had to speak to him about it, actually.'

Janet nodded. She returned her attention to the ma.s.s of wires sticking through the hole where the keypad had been, on the wall by the door. Most of them were still connected. It was just a case of replacing the few wires the Doctor had ripped out or moved to other connections. She did not think she could duplicate his sabotage. But she could repair the system, she was sure.

'Anything else? Anything at all?' the Doctor asked.

Sheldon shrugged. 'My fingers stopped growing,' he said.

'Started again now.' He held up his hand to show. His fingers ended in gnarled nascent knuckles.

'When you got the injection? From Janet?'

He nodded. 'That's when my head cleared.'

'Good. Excellent.' The Doctor jumped down from his perch on the stool and started pacing up and down. 'Check my logic as I go, would you?' he asked. 'What's that?'

'Sorry?'

The Doctor had stopped in front of the tall metal cabinet by the fridge. He pulled the handle and the whole frame of the cabinet rattled in response. 'Hmm,' he muttered. 'Important enough to be kept locked. Where's the sign about the leopard?' he wondered.

'You mentioned logic,' Sheldon reminded him.

'Oh, yes. That.' The Doctor gave a huge pull at the cabinet's handle and the door screeched open. Inside, mounted on a rack, were four rifles. 'Interesting,' the Doctor decided as he took one out.

'Guns?'

'Dart guns,' the Doctor told him. He pulled a clear plastic container from a shelf in the cupboard and angled it so that Sheldon could see the tufted end of the darts.

'Oh yes,' Sheldon said. 'I remember those. Anaesthetic darts.'

'You can load them with any fluid,' the Doctor said. 'Less risky than creeping about with a syringe, wouldn't you say?'

'What did you have in mind?'

The Doctor replaced the gun and the darts and turned to face Sheldon. 'First dosage,' he said. 'It gave you the initial infection. Though you didn't know it. Insidious, slow, creeping progress. Right so far?'

Sheldon agreed.

'Let's not worry how rapidly it actually a.s.sumed control or went through the generations,' the Doctor said. 'What strikes me as important is that a second, concentrated dose had such an interesting effect on you. It seemed to counter the initial infection.'

'But retained its own healing qualities,' Sheldon pointed out.

'You mean it pulled your mind together?' The Doctor nodded. 'True. Very true. Yet the physical healing that was already in progress was halted.' He tapped his lips with a finger. 'Packwood said that the initial generation of Denarian had no effect on the brain. That comes with the later evolution of the material.'

'So?'

'So...' The Doctor continued to consider. He was pacing again, finger still on his lips. He stopped in mid-step and whirled. 'A battlefield,' he said. 'Don't you see?'

'No, actually, I don't think I -' Sheldon broke off. 'Or maybe I do,' he admitted. 'Are you saying that the two batches of material battled it out inside my body?'

'Exactly. That's why your fingers stopped healing. Until the battle was over.'

'But which material won?'

'Oh, the new batch,' the Doctor told him. 'It healed your mind. Did it straight away.'

'No battle to be fought there,' Sheldon said slowly.

'Because the initial material I was infected with didn't get to my brain.'

'Exactly.' The Doctor sat himself down on the stool again.

'But suppose, just suppose for a moment, that the second, more advanced batch of Denarian saw the initial infection as just that - as an infection.'

'Something to be fought.'

The Doctor nodded. 'It purifies the body, makes it as healthy as it can. It saw the alien material as an infection rather than as its daddy. Provoked a sort of allergic reaction to it. Like scorpions stinging each other to death. Don't realise they're relatives.'

'That's possible,' Sheldon agreed. 'There's no sentient or intelligent control of the process. It affects the brain later, we know. But within the host systems, the body, it just sort of gets on with it.'

'What we need,' the Doctor said slowly, thoughtfully, 'is a batch of first generation material that will defeat the Denarian within the infected people's bodies. It doesn't affect the brain, so the side effects would be confined to the healing process, to what you originally intended.'

But Sheldon was shaking his head. 'No, you're forgetting, Doctor, the original material won't go near the brain, so it won't destroy the Denarian there. It won't have any effect, won't matter which batch wins out inside the body. The brain isn't a battlefield, remember?'

'So we need a hybrid, perhaps.'

'What we need,' Sheldon said, 'is a form of Denarian that will defeat the existing infection and cure the host, and then do nothing more.'

'Maybe if it's exhausted and dies in the battle, just as it wins,' the Doctor said. 'Or just peters out somehow. A limited life span, perhaps.'

'Fight fire with fire that then burns itself out,' Sheldon said quietly. 'Can it be done?'

The Doctor was on his feet, rubbing his hands together briskly and surveying the equipment on the workbench in front of him. 'I have absolutely no idea,' he confessed. 'But it should be fun finding out.' He turned to face Sheldon. 'Could you give me a hand?' he asked. 'Er, not literally of course.

Though a small blood sample might come in...helpful.'

The fog had lifted and the stars were clear points of light sparkling like ice in the cold night air. Peri lay staring up at them for a while before she realised that she could not move.

On the branches of a nearby tree she could see a line of seagulls watching her, and her blood ran cold. But they made no move to attack. Didn't even seem curious.

For a while she thought she might be dead, staring up through lifeless eyes at the eternity of the firmament. Then she decided she had broken her neck and was paralysed. But she could feel. She could feel the ground cold underneath her, the rough gravel beneath her hands and head. There was a dull ache from her face and her legs, but she could tell - somehow - that nothing was broken, that the skin was healing already over the scratches and tears in her face.

Then Liz Trefoil's freckled face tilted forwards into her view, blotting out the stars. Peri felt herself rising to her feet, taking Liz's hand, walking with her across the road and up the cobbled street towards the quay.

It was like a dream, a sleepwalk. She had no control, her mind already muzzy, vision blurred and gloomy. She was barely aware of what was happening as Liz handed her down into the small boat. It bobbed beneath her, but Peri did not know it. Her mind was cold and numb as Liz took a pair of oars and began to row mechanically, efficiently, across the water.

A minute later, Peri reached down for the second pair of oars. She knew she was doing it, was aware of the effort and the sensation, but she could not stop herself. Slowly, inexorably, helplessly, she was making her way back to Sheldon's Folly in a body that was no longer her own. She screamed, and no sound came. Her mouth was twisted into a smile, but her mind was in tears.

The keypad was dented from where the Doctor had forcibly removed it from the wall earlier. Unable to get it to slot back into its housing, Janet let it dangle from the wires attached to the back of it. She stepped aside and gestured for Sir Anthony to key in the numeric sequence.

'I hope you're right about this,' she told him.

'Don't worry,' he replied. 'Whatever happens, they're not going anywhere. They come out eventually, or they starve.

Simple.'

He keyed in the sequence.

Sheldon sat with his head buried in his hands as the Doctor encouraged the centrifuge.

'Come on, come on,' the Doctor hissed as the mechanism continued to spin. His fingers tapped impatiently on the worktop. 'Are you all right over there?' he called across to Sheldon.

'Not really.' Sheldon's voice was strained and m.u.f.fled by his hands. 'But you carry on.'

'Almost there,' the Doctor said. 'I think.' The centrifuge was slowing as it came to the end of the timed cycle the Doctor had programmed in. 'All we need is just a little -'

The Doctor broke off as the ma.s.sive door to the laboratory swung slowly open.

'Time,' he finished. 'Pity, that.' He reached out and opened the centrifuge, carefully lifting out the test tubes inside, keeping his eyes fixed on Sir Anthony Kelso and Janet Spillsbury as they walked stiffly into the room. Behind them was a small group of villagers, several carrying spades. Rogers stood beside Bob Trefoil, who was hefting a sledgehammer.

Sheldon slipped from his stool before Sir Anthony reached him. He stumbled across towards the Doctor. Janet stepped briskly towards him, grabbed his shoulder and pulled him back, dragging, pushing. Sheldon slipped to the floor as she shoved him towards the wall, where he remained in a bundle of ragged clothing.

The Doctor pushed one of the test tubes into the wooden rack. He held the other one up, as if in defence, as Janet continued towards him. Behind him, on the workbench, the Doctor fumbled for the syringe. He had cleaned it out thoroughly - all he needed to do now was fill it. If he had time.

Janet ran at him, hands outstretched, fingernails pointing at the Doctor's face as she sprang. He took a step backwards, caught a foot in the legs of a stool, went head over heels, managed - somehow - to right himself and ended up sitting with the test tube still raised, undamaged. The syringe clattered to the floor, its plastic body jumping then rolling away, out of reach even as the Doctor leaned forward and scrabbled for it.

His fingers slipped on the cylindrical surface, grazed it desperately as he tried to reach it. As Janet's foot connected with the side of the Doctor's face, he changed his tactic, sending the syringe spinning away from both of them, towards the far wall. Towards Sheldon.

Sir Anthony was standing, hands on hips, as he watched Janet kick the Doctor again.

'You're an infection,' he said as the shoe connected and the Doctor cried out. 'A blemish on an otherwise perfect world.'

The Doctor was shifting round, spinning on the floor, struggling to crawl away as she kicked him a third time. 'Is that what you think?' he gasped as he rolled clear, gaining impetus from the blow. Still he held the test tube aloft, like a talisman. To no effect.

'A canker that must be cut out,' Sir Anthony went on. 'An open wound that needs to be seared, closed, sutured.'

'We'll see,' the Doctor said indistinctly. He wiped his mouth with the back of his free hand, pulled it away bloodied and damp. 'We'll see,' he said again. Then: 'Here, catch!'

Sheldon seemed to come to life as the test tube arced through the air towards him. Suddenly he was moving, rolling, hand outstretched. He caught the gla.s.s phial cleanly in his stumpy new-grown fingers, kept rolling, gathering the syringe as he went.

Sir Anthony was turning towards Sheldon now, as if realising he was there. Bob Trefoil raised the sledgehammer.

The Doctor was a blur of motion as he managed to catch hold of Janet's leg as she lashed out at him again. He held on, twisted her foot viciously and sent her skidding across the room to connect with Trefoil. The sledgehammer went flying, bouncing on the workbench and taking equipment and gla.s.sware with it as it skewed across the top. It caught the test tube rack, shook it, rolled it to the very edge of the workbench.

The wooden rack teetered on the edge. And fell.

The Doctor just made it, his hands underneath the rack as it fell. He caught the rack upside-down. And the test tube fell neatly from it. It bounced on end on the floor, rose, spun, and was gathered by the Doctor's other hand as he dropped the rack.

He looked up in triumph, to see Janet's face close to his, teeth exposed in a grotesque parody of a grin as her long nails reached down towards his eyes.

She blinked, a sudden reaction as she stiffened, froze.

Behind her, over her shoulder as she dropped away, the Doctor could see Sheldon. The syringe was still sticking out of Janet's arm as she fell. The Doctor grabbed it as it pa.s.sed him. Empty.

And Sir Anthony was closing on Sheldon now. The Doctor was pushing himself away across the floor, juggling syringe and test tube. The rubber bung in the top of the test tube was in his teeth as he pulled it out, spat it away, pulled at the plunger of the syringe and felt it pulling oh-so-slowly from the body.

Rogers and Trefoil were helping Sir Anthony as they dragged Sheldon to his feet. Janet was scrunched up in a lifeless ball under the workbench. The other villagers were advancing slowly on the Doctor as he struggled to steady his hand, to pour some of the liquid into the syringe. Not too much, save some for later.

He pushed the plunger back into the body of the syringe, turned it upright and expelled a tiny drop of the viscous liquid.

He smiled in satisfaction. But the smile turned into an anguished cry as Janet Spillsbury lifted the syringe from his hands and stepped away from the Doctor.

'It's finished,' she said, and the venom in her voice was unmistakable.

The boat b.u.mped gently against the jetty as Liz threw the painter over a wooden pole. She stepped easily on to the wooden boards, not waiting to see if Peri followed.

The two of them walked, in step, towards the house. The moon was shining bright and clear now through the skeletal limbs of the bare winter trees. Somewhere in the distance an owl hooted, a satisfied, fulfilled cry. Like calling to like across the wooded island.

There was a line of villagers standing across the pathway to the house. As Peri and Liz approached, they stepped aside, made room. Liz Trefoil joined the line, standing beside Jed, from the farm.

Further along the line, Hilly Painswick watched impa.s.sively as Peri took her place in the ranks of the Denarian. Then she turned her pale inhuman eyes back towards the jetty.

'Finished!' Janet Spillsbury repeated as she stepped across to Sir Anthony Kelso and stabbed the syringe into the back of his neck.

He shrieked, an inhuman wail of pain, hand clutching, struggling to pluck out the needle. It clattered to the floor, and he sank to his knees after it.