'Doctor? Stoker? Are you there?'

There was no reply. It was so cold she couldn't breathe properly and she felt her heart racing. She turned around and realised that she couldn't even see the cavern exit any more.

She tried the radio. 'Doctor? Where are you? Are you receiving me?' The comlink gave a buzz and the Doctor's voice responded, 'Tegan! We've had to go deeper into the caves. We're heading for the lab.'

'It's really dark in here,' Tegan said plaintively. 'I'm scared, Doctor!'

'I know,' the Doctor said, and Tegan was surprised at how strained his voice sounded. Almost as as if he, too, was scared: but that was impossible. Wasn't it? When the Doctor spoke again, it seemed to be with some effort, 'The Dark is attempting to coalesce. It can only achieve complete, corporeal existence in total darkness. It's absorbing all the light energy from the cavern to try and create the right environment for it to enter this dimension.' if he, too, was scared: but that was impossible. Wasn't it? When the Doctor spoke again, it seemed to be with some effort, 'The Dark is attempting to coalesce. It can only achieve complete, corporeal existence in total darkness. It's absorbing all the light energy from the cavern to try and create the right environment for it to enter this dimension.'

'Doctor, I don't know what you're talking about. Just tell me what to do!'

'Try and get back out of the cave. If you can't do that, then follow us down here. But be quick!'

'But what about Nyssa?'

The radio crackled and went dead. Tegan looked nervously all ground her. She couldn't see a thing. But she could hear a distant, anguished wail building slowly behind her, like the cry of a thousand grieving parents.

The Dark was coming for her.

'What are you looking for, for grief's sake?' Stoker demanded.

She was watching the Doctor yank open locker after locker in Ravus Oldeman's lab, scrabbling through the contents of each with increasing exasperation.

'Something. Anything. I don't know,' he said. He opened another locker and swept the contents out onto the floor.

Medical supplies scattered across the tiles. He instantly dropped to his knees and started to hunt through them.

'Why didn't you answer Tegan? What about Nyssa?'

The Doctor continued with his frenetic search. 'There isn't time to think about anyone else at the moment.'

Stoker felt a chill run through her bones. That didn't sound like the Doctor at all.

'Got it!' the Doctor said triumphantly, holding up a small medical injector.

Stoker knelt cautiously down by him. 'Neurolectrin?'

The Doctor's eyes gleamed. 'Just the ticket.'

'What for? I thought it was only used for treating people coming round from suspended animation.'

'It is!'

And you need it because..?'

The Doctor opened his mouth to reply and then stopped.

He looked uncertainly at the injector and then at Stoker. His eyes had the look of a small boy struggling to find the right answer to a simple question. 'I need it because... I need it because...' He faltered. 'I don't know why I need it. Do I need it?'

'Get a grip, Doctor. The Dark must be messing with your head again.'

He nodded. 'Yes, that must be it: the Dark. I was thinking about Professor Oldeman. About how he died.' The Doctor sank into a sitting position and hugged his knees. He looked so helpless, and that was scaring Stoker more than anything.

'All his blood, just running away like that. His whole life...'

'Don't dwell on it, Doctor,' Stoker told him firmly. 'It wasn't your fault.

'No, but still...' the Doctor stared into s.p.a.ce. 'All that blood.'

'Many people have died here. All of them nastily. But now is not the time to start brooding about it!'

'Are you sure?' The Doctor gazed intently at her. There was something odd in those blue eyes. 'Think about it: what happens when someone dies? What actually happens happens to them? To the to them? To the person. person. Where do they go?' Where do they go?'

'No one knows the answer to that one, Doctor.'

'They should.' His voice was small and frightened.

'Someone should find out. Then we'd all know.'

'You're getting irrational, Doctor. More than that, you're starting to get on my nerves.'

He looked at her again, and his eyes seemed to gain focus for a moment. 'Hit me,' he said.

'What?'

He scrambled to his feet. 'Hard as you can. Quickly!' He barked the last word like an order and Stoker jumped up.

Then she slapped him across the face.

'Harder!'

She slapped him again, and it felt rather satisfying.

'Again!'

This time his head jerked to one side with the force of the blow.

'Better!'

'Mind telling me what this is supposed to achieve?'

Stoker asked, hitting him again. 'I mean, besides making me feel a whole lot better.'

'I'm trying to use pain to weaken the Dark's mental grip; give me something else to think about.'

'Right,' said Stoker. She made a fist and punched the Doctor hard enough to send him sprawling across the lab. He smashed into a workstation and flipped over the desk, landing heavily on the other side.

'That do the trick?'

It took a couple of minutes to bring him back to consciousness. When his eyes finally opened Stoker said, 'Sorry. Didn't mean to hit you quite so hard.'

He sat up and rubbed his jaw. 'Did you ever meet John L Sullivan?'

'No, but I was the bar-room brawl queen at college,' she said. 'How d'you think I got this?' She pointed to her crooked nose and the Doctor looked a trifle alarmed. 'Don't worry, your boyish good looks are unmarked. Relatively. At least you sound a lot better.'

He got up, with a little help, and leant against the desk. 'I am a lot better. For now. But it's getting more and more difficult to resist the Dark's mental pressure. It's getting stronger and I'm getting weaker.' He sat down heavily in a chair. 'I don't know how I'm going to stop it.'

'You'll think of something.'

'I wish I shared your confidence in me,' He looked absently at the little injector of neurolectrin that was still lying on the table in front of him. 'I wonder...'

'What?'

'Neurolectrin helps reverse synaptic decay. It acts like a sort of lubricant for rusty brains.' He picked up the injector.

'What if I were to take this dose? Would it help me resist the Dark?'

Stoker said, 'I've no idea. But I've never liked drug abuse.

Apart from alcohol. And tobacco. And caffeine. And...'

The Doctor prepared the injector and rested it against the skin of his throat. 'It's worth a try,' he said.

He sat there with his finger on the activator, unmoving.

Stoker watched him and waited. 'Then why don't you try it?'

'Because I've just had second thoughts: what if it has just the opposite effect? The neurolectrin might make it easier easier for the Dark to control me.' He pulled the injector away and stared at it as if it was a live scorpion. 'It's doing it again!' he cried. 'Trying to trick me! Giving me ideas that aren't my own!' for the Dark to control me.' He pulled the injector away and stared at it as if it was a live scorpion. 'It's doing it again!' he cried. 'Trying to trick me! Giving me ideas that aren't my own!'

Stoker suddenly stiffened and glanced around her. 'The light's fading.'

The Doctor leapt to his feet, looking around the lab.

'Photon absorption. The Dark's trying to get in here.'

The shadows lengthened around the laboratory like expectant shrouds. 'What now?' Stoker asked.

'We run!'

Tegan ran down the steps from the main cavern and found the lab complex in darkness: cold fear gripped her stomach tightly as she stepped inside. She could see the tell-tale lights on the computers and work stations, glaring like little red eyes from the shadows, but that was all.

The Doctor and Stoker had gone.

She crossed through to the main section, breathing heavily.

Darkness behind her, darkness ahead: she knew she was being surrounded. Cut off.

She could feel its malevolence in the turgid air all around her, like a swelling under the skin of reality.

In front of her a shadow bulged, growing out of the blackness, full of hate and spite. It hissed and writhed into a formless shape, filling the lab and almost blocking the way ahead. With a rush of desperate courage, Tegan flung herself past it and through the doorway beyond. She felt its cold, moist kiss as she slipped past and shuddered.

She stumbled down the short flight of steps into the stasis-tank chamber. The room was dark and empty. Where the h.e.l.l was the Doctor? Tegan yanked the radio out of her pocket. 'Doctor! Where the h.e.l.l are you?'

The comlink bleeped and the Doctor skidded to a halt. He fumbled through his coat pockets and eventually produced the radio. 'h.e.l.lo? Who is it?'

'It's me! Tegan!' crackled a voice. 'Where are you?'

The Doctor hesitated. 'Er, well, I'm here. Where are you?'

There was an angry squawk from the comlink and the Doctor glanced helplessly at Stoker.

Stoker grabbed the radio. 'This is Stoker. We've had to get out of the lab complex because the Dark's trying to manifest itself there.'

'You're telling me!' Tegan's voice was distorted with fear.

'It's right behind me! What's wrong with the Doctor? Why can't I speak to him?'

'The Doctor's not, ah, feeling very well. At the moment.'

Stoker's mind was racing. 'I think the Dark's affecting his mind.'

Tegan's voice dropped to an anguished whisper, 'What am I going to do?'