'Come on!' the Doctor shouted. 'Don't dawdle.' He was gesturing for Sir Anthony to start up the path, not to wait for them. In a moment the three of them were staggering back up the steep path. Several times Peri lost her footing and slipped back. Once she managed to get her balance, once the Doctor caught her hand as she flailed and windmilled. Once she fell to her knees with a cry.

At the base of the cliff, the fishermen were starting up the path after them.

'What do they want?' Sir Anthony asked breathlessly.

'You ask them!' Peri suggested.

'I think we should shelve that question until we're in a rather less precarious and perilous situation, if you don't mind,' the Doctor said.

When they reached the top, Peri asked, 'Where now? Back to the cottage?'

The Doctor shook his head. 'I still think our best option is to get to Sheldon's Folly and ask a few pertinent questions of the people there.'

'The quay,' Sir Anthony suggested.

'Good idea,' the Doctor agreed. 'We can borrow Old Jim's boat. I don't think he'll object under the circ.u.mstances.'

They had pretty much got their breath back by the time they reached the quay. There was no sign of the dead fishermen following them, but none of them wished to a.s.sume that they had given up and gone away.

Most of the boats were gone, out fishing as their owners made the most of the good weather. As they approached the small quay, Peri could make out the shape of a figure sitting hunched on the ground close to the main cl.u.s.ter of small boats. 'Is that Old Jim?' she asked the Doctor.

'Let's see, shall we?'

He was facing away from them, so it was not until they were quite close that the Doctor stopped. 'No,' he said, 'that's not him.' He raised his voice and called. 'Excuse me, could you tell us where Old Jim is? We'd rather like to borrow his boat if it's not too much -' He broke off as the figure slowly turned towards them.

'Trouble,' Peri said.

It was Dave Madsen. What was left of him. He was staring at them now, through what was left of his eyes. Slowly, as if afraid more of his body would fall away, he got to his feet.

Then he started shuffling slowly towards them.

'You said you thought Madsen was involved,' the Doctor said quietly to Sir Anthony. 'I'd say that was a pretty accurate guess.'

'What do we do now?' Peri asked. Her voice was husky and trembling.

'Run?' The Doctor suggested. He looked expectantly at each of the others. They both nodded. 'Run,' he decided.

She was breathless now, running for all she was worth, desperate to get back to Sheldon's Folly to tell Packwood what was happening. What had gone wrong.

As she approached the quay, Janet skidded to a halt. She could see people in front of her, at the quay. People running.

And they were running from...

She turned to watch the three people running along the quay. One of them was the Doctor, another the girl she had met at the pub. She did not recognise the older man, the one with the stick. And partly because they were heading towards her boat, and partly because she could see what they were running from, Janet ran after them, shouting.

Peri turned at the sound. She heard her name, the Doctor's name being called. She saw Janet Spillsbury running towards them, waving, shouting something about a boat, about having to get to the other island. She saw the b.l.o.o.d.y mess that had once been Dave Madsen turn towards her as she ran. Then she saw Janet Spillsbury catch her foot on a cobble and fall heavily to the ground.

She sat there in the road, clutching her ankle. And then Peri saw the silhouetted shapes of the two fishermen from the bay as they rounded the corner behind Janet.

Peri screamed. The Doctor and Sir Anthony both shouted.

Janet heard them, turned to see the fishermen close behind her, leaped to her feet and limped frantically towards the quay.

Even as she made her way towards the Doctor and Peri, Madsen moved to cut her off.

The Doctor was off and running now, heading straight for Madsen, cannoning into his back, shoulder down like a rugby player. Madsen buckled and went flying. The Doctor grabbed Janet and pulled her up to the quay.

'Boat,' she gasped as she limped along. 'My boat. Over there.'

It was a motor boat. There were two boats tied up close together. One had a grimy outboard motor pitched forwards out of the water. Janet hobbled towards it, gaining speed as she got into the rhythm of moving. Sir Anthony was ahead of her, throwing in his stick and climbing carefully down, trying not to rock the boat too much in the water. He turned to help Peri in after him.

Janet was barely limping at all by the time she and the Doctor caught up with Peri and Sir Anthony. They both jumped down into the boat, so that it shook alarmingly. Sir Anthony was already untying the painter. The Doctor pushed away from the quay with an oar and Janet pulled at the cord on the motor.

Madsen had recovered immediately and was already close to the boat. The two fishermen were not far behind. They clambered down into the next boat as the motor whirred. But did not catch. Janet tried again. One of the fishermen had the oars now. The Doctor was rowing too, shouting at Janet to get the motor started. But his attempts to row were erratic and ineffective compared with the dead fisherman's steady, strong pull. The rowing boat was gaining on them.

Yet again Janet heaved at the cord of the outboard. This time it coughed into uneasy, smoky life for a few seconds. But then the motor died again, and Janet gave a sob of annoyance and anguish.

'Let me,' Peri offered, trying to move round to grab the cord.

'I can do it,' Janet insisted.

The fishermen and Madsen were almost within reach now.

Madsen was standing in the prow of the small boat, reaching out towards them with flailing arms, a few streaks of blond still visible in the matted red tangle of what was left of his hair.

'I just thought -' Peri settled back down.

'I can do it!' Janet screamed at her, and heaved at the cord.

The motor spluttered into uncertain life, choking and coughing as it belched out dark smoke.

A dead hand plucked at Janet's hair as she let the cord back in and adjusted the throttle.

Janet dipped the propeller into the water as Madsen grabbed at her. The back of the boat bit low into the sea and the prow rose as it shot forwards. Janet cried out, hand to head as she sank down on the bench in the stern of the boat. Behind them Madsen was left clutching a handful of jet black hair.

'How's your ankle?' the Doctor asked. 'That was a nasty tumble you took back there.' He leaned forward to look as Janet raised her leg and pulled off her shoe.

'It feels fine now,' she said. Her voice was shaking.

'Not even bruised,' the Doctor murmured. 'You're lucky.'

Janet was staring at her foot. Slowly, she slipped her shoe back on. Then she looked round, feeling along the wooden edge of the boat.

'What is it?' Peri asked. 'What do you want?'

'Something sharp,' she replied, her voice quiet but tense.

'Ah.' She had found a screw sticking out of the casing of the outboard motor where it had worked its way loose. The head was standing proud of the metal surround, angled and rough.

Peri watched in astonished disbelief as Janet Spillsbury carefully put the palm of her hand against the screw, then suddenly, violently, dragged her hand downwards. Her face was contorted with pain as she cried out, immediately clutching her hand. There was a smear of blood down the casing of the motor. The screw was a crimson point in the middle of it. Janet's hand was ripped open, blood welling up and dripping to the floor of the boat.

'I'll get you a tissue. A hanky.' Peri was feeling in her pockets. 'Something to bind it up with.'

Both the Doctor and Sir Anthony were watching Janet's hand bleed. 'No, Peri,' the Doctor said quietly. 'That won't be needed.'

'What?'

'Look.' He nodded at Janet's hand.

She held it up for Peri to see. 'You think I was lucky?' She gave a sad, ironic laugh. 'Quite the opposite.' The blood had congealed already over the hand. It was flaky and dry now. As she spoke, Janet rubbed it away with her other hand, to reveal the unblemished palm beneath. 'I'm infected,' she said, her voice husky and dry. 'Oh G.o.d, I'm infected. Like the others.'

The Doctor sat himself next to Janet on the back bench and took hold of the handle for the motor, angling it slightly so that they were headed directly for the dark outline of Sheldon's Folly. 'It will take us a few minutes to get to the island,' he said. 'And I've got nothing much else to do. So why don't you tell us all about it?'

Janet looked up at him. Her eyes were wide and moist with welling tears. 'You wouldn't understand.'

'Oh, I think you'd be surprised.' The Doctor looked into her eyes. 'It came back on the Gatherer s.p.a.ce probe, didn't it?

Some form of genetic material. Alien genetic material. And you thought you could make use of it.' He shook his head sadly.'But now you're beginning to realise that it's making use of you.'

'They called Christopher in as soon as they knew it was genetic. That was his thing. The rest of the stuff was ice and dust. Bits of asteroid and the frayed ends of a long-forgotten comet. But sample three-zero-seven, that was the big one. I didn't join until much later of course, so I missed the initial anxiety, the containment problems, the decisions and heart-in-mouth philosophising. Trying to merge what they had with the DNA computer work that was already so far advanced. They were up and running by the time they needed me. Caught up in the excitement and the potential of it all. The implications.

When you're that involved, that close, that enthusiastic, it's difficult to realise that things are going wrong.

'Especially when you're changing the world.

'I knew Chris already, of course. I think that was why I had to be on the team, so he could talk to me about it. The ESA liaison stuff was hardly useful by then. He called it Denarian. From DNA, you see. It was pretty clear from the first what the potential was. A universal cure-all was how they explained it to me. I laughed, of course. And then they showed me the test results. The regenerated tissue, the repaired nerve endings, skin grafts. It's funny, it never occurred to me then, only now, now that I see the progression. We never stopped to ask how the stuff knew when to stop.

'I was thinking that in the pub. Just now. This afternoon.

As I sat there. You know the old joke about how a thermos flask keeps hot liquid hot, and it keeps cold liquid cold. But how does it know know? I guess it was like that. We just a.s.sumed that was the way it was, the way things happened. Never mind how. Until now. Until I saw what it was doing to Bob Trefoil's grey hair. How it was a.s.suming control, slowly but surely taking over. And then it struck me. The basic instinct of any living thing. And it is living, you know. When his hand spasmed, when he lost control. That's how it does it.'

Peri frowned. 'Does it?'

'Survives,' the Doctor said quietly. 'The basic instinct of any living creature is survival.' He let out a long misty breath.

'You know, it's more like the story of the little girl who says she knows how to spell "banana", but she doesn't know when to stop.'

Janet was nodding. 'That's right. Partly it just doesn't know when the human host is dead. And why would it care? It keeps on repairing the tissue, mending the bones. Not the brain of course, we had checked that it doesn't affect the brain. Not in the second generation.'

'The second generation?' Sir Anthony asked.

'But now it's jumped again. Taken another evolutionary step forwards. To a third generation. First it realised that to survive it had to keep the host healthy, look after its own environment. That was what the second generation was about.'

'And now?' the Doctor asked, his face grave.

'Now,' she said, 'it's realised that to survive it needs to take control of the host.' She looked up at him, then at the others. 'Total control.' Then she looked back at the wet boards in the bottom of the boat. 'And I'm in the second generation.'

The island was looming in front of them now, the rocky coastline jutting ragged and forbidding out of a shroud of mist.

'The first generation,' the Doctor said slowly. 'I a.s.sume there is one. An incubation period?'

Janet nodded. She was staring straight ahead at the coastline. She did not look at him. 'Yes.' Her voice was a strained whisper. 'Cultivation of the raw genetic material.

Within an...incubator. That's right.'

The Doctor angled the boat so that it was heading for the wooden jetty, a boarded pathway into the depths of the mist. 'I don't think it's a conscious thing,' the Doctor said as he brought the boat in. 'I don't think we're up against a planned invasion here. We know from the herding instinct, the way the children a.s.similate behaviour and abilities, that there's a collective consciousness of some sort. But it certainly isn't conscious in any way that we would understand. It's just a life form trying to survive the only way it knows. Somehow this whole process is its life cycle and you've got plugged in as the necessary symbiotic host.'

'Thanks,' Janet said as she allowed the Doctor to help her out of the boat. Her voice was rich with sarcasm. 'That makes me feel a lot better.'

Sir Anthony stepped on to the jetty and tied up the boat as the Doctor cut the engine.

The Doctor said nothing for a moment. He gave Peri a hand up, then turned back towards Janet. His face was dark with anger. 'It isn't our fault what has happened here,' he said.

'It wasn't us who brought this thing, this scourge to the islands. It wasn't our negligence or l.u.s.t for scientific advancement that infected you. And it wasn't the islanders either. But they are the ones who are dying because of it.

Remember that.'

'I'm sorry.' Her voice was quiet. She looked away, biting her lip.

'You may not like me,' the Doctor went on, his voice also quieter now. 'You may not like what I do, what I shall have to do. But I'm here to help you, and it may be that I'm the only help you have.' He turned away, suddenly bright and cheerful again, marching along the jetty and drawing the others after him in his wake. 'I wonder if Mr Packwood has the kettle on,'

he asked the cold surrounding air. 'I think we could all do with a cup of tea to calm our nerves.'

They made the rest of the journey in silence. The strange, misshapen house loomed out of the mist as they approached, seeming to lean down towards them. Peri shuddered at the sight, but the Doctor seemed, if anything, to become even more animated and enthusiastic. As if he were relishing the imminent confrontation.

'Right,' he p.r.o.nounced, skidding to a halt on the wet gra.s.s and turning to face the others. 'A few rules then. You,' he pointed at Sir Anthony, 'are Sir Edward Baddesley, as we all know.'

'If we all know that,' Janet began, but the Doctor raised his finger and his eyebrows to cut her off.

'Nothing is as simple or straightforward as it seems,' he said darkly. 'You,' he dropped the finger so that it pointed at Janet, 'tell me if you begin to feel the slightest suggestion that you are losing control of your body or your mind. Anything at all - dizziness, blurred vision, even a tingling in your feet.' He turned finally to Peri. 'And you...'

'Yes, Doctor?' She stood up straight and thrust her chin out bravely.

'You be very careful, and do as you're told,' he decided.

'Come on.' And with that he turned and headed towards the front door of Sheldon's Folly.

The door swung open even before they reached it. Logan Packwood was standing there, his arms folded, his head tilted slightly to one side as he watched them approach.

'Sir Anthony Kelso,' he said as they arrived. 'How nice to see you again. Come to check up on us, have you?'

'So much for that part of the plan,' the Doctor said with evident disappointment. 'But yes, I think some checking would be in order, don't you? Your precious alien genetic material is out of control. We have to put a stop to this now.'

'Indeed we do,' Packwood agreed. 'Indeed we do.' He stepped aside, as if to let the Doctor into the house.