No expedition had ever returned from that terrible planet. Incredibly cunning and ferociously powerful, the mind parasites drained the energy of any intelligent beings they encountered.

The Master had brought one here, imprisoned in a force-field, disguised as a Machine, to serve his own evil purposes. Judging by his eagerness to pa.s.s on the care of the creature to the Doctor, it was rapidly growing too strong for him.

Jo wandered back to the window. 'Doctor, look. It's the Master. He seems to be leaving.'

Peering cautiously through the window curtains, the Doctor saw the Master standing by his black limousine, having a last word with Mailer who, in turn, seemed to have just arrived in a Black Maria, with a number of other convicts.

The Master got into the limousine, and it drove away.

'Right,' said the Doctor. 'Now's our chance!'

Jo nodded eagerly. 'We can slip out by the hospital and through the rear courtyard.'

'I didn't mean our chance to escape, Jo. I meant my chance to get into the Process Chamber, and destroy the creature in the Machine.'

'I see.'

Jo tried to hand over the gun she had taken from Vosper, but the Doctor waved it away.

'You keep it, Jo, you're trained to use those things. They only make me nervous.'

They hurried out.

After a good deal of banging and shouting, Charlie and Vosper had been released by a pa.s.sing fellow prisoner.

Charlie had promptly got hold of a gun, and gone off in search of the Doctor, the blow with the tray still very fresh in his mind, with a headache and a b.u.mp on his head to make sure he didn't forget.

His search of the corridors and courtyards had been fruitless, and now he was returning towards the Process Chamber in case the Doctor had taken refuge there.

Suddenly the air in front of him seemed to ripple with a blood-red glow. It cleared, and there was the Keller Machine in the middle of the corridor. It was throbbing angrily.

Charlie stared in astonishment and screamed as the mind parasite sucked his mental energy, his very life force, out of his mind and body with a dreadful voracious hunger.

Charlie fell dead, no more than an empty husk.

The air shimmered and the Machine disappeared.

A few minutes later the Doctor and Jo came hurrying along the corridor and almost stumbled over the body. The Doctor knelt to examine it.

'Look at his face,' whispered Jo. 'He must have been terrified, just like the others.'

The Doctor rose. 'Yes. Perhaps the creature has learned to move.'

'How?'

'Teleportation. Come on, Jo.'

They hurried down the corridor and went into the Process Chamber. The Machine was gone.

'You were right, Doctor,' whispered Jo.

A voice said, 'Drop the gun, girly.'

Vosper and Mailer were standing in the doorway. Both were armed.

Mailer raised his heavy automatic. 'I think I've had just about enough of you.'

The Doctor began talking, in the hope of distracting Mailer. 'Maybe so, but before you pull that trigger, I think I should warn you '

Suddenly the Doctor broke off.

He stared over Mailer's shoulder, eyes widening in horror.

'What's the matter with you?' snarled Mailer.

'Look behind you!'

'I've heard that one before.'

'Look behind you!' whispered the Doctor.

Such was the urgency in his voice that Mailer turned.

The Keller Machine was materialising in the corridor behind Vosper.

Hearing the throbbing, Vosper swung round, and fired a shot at the Machine. It sucked him towards it like a magnet, held him writhing and screaming for a moment, dropped the discarded body and disappeared.

The Doctor swept Jo into hiding behind the console, while Mailer had flattened himself against the wall just outside the door.

When the Machine vanished, Jo and the Doctor emerged cautiously from hiding.

There was a throbbing sound, a blood-red glow, and the Keller Machine materialised in front of them...

12.

The Escape For a moment Jo and the Doctor stood motionless.

The Machine was throbbing, and they could feel its power, drawing them towards it.

Suddenly Mailer sprang through the doorway. One hand was clutching his forehead, the other held an automatic. Face twisted with terror, he fired wildly at the Machine, then turned and fled down the corridor.

The Machine shimmered and disappeared.

'What happened, Doctor?' gasped Jo. 'Why did it just go away?'

'I don't think we can have tempted its appet.i.te sufficiently.'

'Why not?'

'I imagine it detected a higher concentration of evil in Mailer. Something to be said for a pure mind after all!'

Jo shivered. 'I think we'd better get out of here before it comes back for us.'

She went out into the corridor, and the Doctor followed.

There was no sign of Mailer or the Keller Machine. Instead they saw Barnham, wandering distractedly towards them in his striped hospital pyjamas and dressing gown.

'Barnham!' exclaimed Jo. 'What are you doing here?'

He gave her his confused, child-like smile. 'I was looking for you. I heard this noise.'

Jo took his arm. 'Well, you come with us. Don't worry we're getting away from here.'

'But I didn't understand.'

The Doctor turned to Jo. 'Come on, we've got to stop the Master from launching that Missile.' He patted Barnham on the shoulder. 'Don't worry, old chap, just you come with us!'

The UNIT helicopter droned over Stangmoor Prison, hanging in the air like a great metal dragonfly.

The Brigadier studied the peaceful scene below.

Occasional blue clad figures strolled across the courtyards, the scattering of parked vehicles looked like toys against the grey stone bulk of the prison...

He spoke into his intercom. 'Windmill 347 to Trap Four. Do you read me?'

A crisp voice came through the headphones. 'Trap Four to Windmill 347 reading you loud and clear.'

The voice belonged to Major Cosgrove, who had been a.s.signed to help the Brigadier on this operation.

Cosgrove was the new academic type of soldier. He was neat, bespectacled, intelligent and quite unbearably efficient. He sat in the UNIT mobile HQ parked a few miles from the prison, listening to the Brigadier's voice.

'Major Cosgrove, reading you loud and clear,' he repeated. 'Any results, sir? Over.'

'I'm over Stangmoor Prison now, Major. No sign of the missile and everything seems quiet and normal.'

'Are you going to land, sir? Over.'

'No, I don't think so. I know I said everything seems normal, but I'm quite sure it's not. I'll continue the recce for the moment. Over and out.'

The Doctor, Jo and Barnham had reached a door that led to the courtyard.

The Doctor opened it cautiously. 'n.o.body about. Come on, Jo, let's take a look outside. Barnham, you'd better stay here.'

They moved cautiously out into the courtyard. Jo leading the way. She heard a droning sound, looked upwards, and immediately began capering up and down, waving her arms. 'Look, Doctor, it's a UNIT helicopter.'

Jo went on waving frantically. Caught up in her enthusiasm, the Doctor started waving, too.

From behind them a harsh voice said, 'All right, you two, back inside!'

Two armed convicts had appeared in the corridor behind them. Reluctantly Jo and the Doctor went back inside the prison.

The bigger of the two convicts, a balding thug called Fuller, jabed them with his rifle to hurry them along. 'I said back inside, come on!'

Suddenly Fuller found the ma.s.sive figure of Barnham looming over him.

'Don't hurt them,' said Barnham in an unhappy voice.

Fuller tried to shove him away with the rifle, but Barnham grabbed the weapon in one enormous hand and shoved it across Fuller's throat. 'I said don't hurt them.'

The Doctor saw that the bemused smaller convict was raising his weapon to fire.

Hurriedly he pulled Barnham away. 'It's all right, old chap. Let's get out of here, shall we?'

Reluctantly Barnham let go of the rifle. 'He was going to hurt you,' he complained.

'It's all right,' said Jo soothingly, and Barnham allowed himself to be led away.

In the Governor's office Mailer, who had somehow managed to elude the Keller Machine, was talking to the Master on the telephone. He was furiously angry. 'What do you mean, we messed things up. You got your missile, didn't you?'

The Master was speaking from a little office, built into one corner of the disused aircraft hangar. Beside him was Mike Yates, unconscious in a chair, his hands tied behind him.

'You left one of the UNIT people alive,' said the Master coldly. 'He followed you here. Fortunately he was caught.'

'Look, mate, I don't care about that,' said Mailer's impatient voice, 'I want you back here at Stangmoor right away.'

'That's quite impossible. I'm busy preparing the missile for launching.'

'I don't care how busy you are,' said Mailer furiously.