Beside the Z-Bomb, a series of hexagonal manhole covers led down to a small nuclear reactor pile which provided the base with light, heating, and power.

The reactor rested on nothing but the solid bedrock of the Antarctic.

Ben, Barclay, Dyson and one of the technicians were easing the bomb on to a trolley in readiness for its removal to the lefthand side of the room.

They looked like s.p.a.cemen in their bulky white anti-radiation suits and perspex head vizors.

'Do not help them.' The Doctor's voice boomed through the loudspeakers.

They looked up at a small monitor screen showing the tracking room. The Doctor had turned to Cutler's console and depressed the PA switch. Again, his voice came over the loudspeakers.

'They mean to use the bomb to blow up the Earth ! '

The PA system abruptly clicked off and, on the monitor screens, they saw the Doctor flung back against the wall with one sweep of the Cyberman's arm. The Cyberleader leant over the console and slammed his fist down. Abruptly, the monitor screen blanked out.

Ben turned to the others, his voice m.u.f.fled through the mouthpiece of the radiation suit. 'Did you all hear that?'

'Of course,' replied Barclay. 'It all makes sense now,' he continued on bitterly.

'We've allowed ourselves to be fooled by them.'

Dyson nodded. 'We just set them up nicely. Cutler was right, wasn't he? We should have used the bomb on them-whatever the consequences.'

Barclay shook his head. 'That might easily have started off something far worse.'

'Worse ! ' Dyson raised his arms as far as his bulky suit would allow. 'We're about to be blown up, along with the entire population of the Earth, and you talk of something worse happening! '

'Give over, mate.' Ben spoke sharply. 'What he means is while there's life, there's still hope.'

But Dyson moved away in despair. 'I've a feeling we've just signed our own death warrant.'

Barclay turned away from the bomb, silent and preoccupied.

Ben looked from one to the other. An idea was beginning to form. 'Half a mo'. I'm beginning to get the drift of all this.'

'Marvellous ! ' said Dyson sarcastically.

'Yeah,' continued Ben angrily. 'Well you might at least listen! I haven't heard any bright suggestions from you two brains!'

Barclay turned back. 'Sorry. Go on.'

'Any idea how strong these Cybermen are?' asked the sailor.

Barclay shrugged. 'A rough idea.'

'Well, they can lift a man like...' Ben looked around and lifted a spanner, '... this spanner, right? They are five, maybe ten, times as strong as we are. They are also pretty advanced geezers, right? Way ahead of us in science and technology?'

Dyson snapped irritably. 'What's all this got to do with it?'

'Plenty. If they're so strong and clever, why do they want us to do the work for them? They could shift this bomb in half the time. What's more, you must have noticed that the Cyberguard always stays outside this room, watching us through that door.' He pointed to the Cyberman's helmet, which was visible through the thick gla.s.s observation panel.

'Why?' Ben asked.

'This is just a waste of time,' mumbled Dyson.

But Barclay grasped his arm. 'No, wait. I see what you're driving at. They use us because they can't handle the bomb themselves.'

'Yeah, that's it!' said Ben excitedly. 'The point is, why? You're the scientist.'

Barclay thought for a moment, then smiled. 'Of course, it's quite clear. Don't you see, Dyson? The reason could be that they are afraid of radioactivity!'

Dyson looked towards the door, and then back at the others. He nodded a little reluctantly. 'Could be!'

'Well don't let's just stand here, let's prove it,' said Ben. 'Let's get this one inside here. See what it does to him. Come on, lie down on the floor.' He turned to the waiting technician. 'You, too. All of us. Play dead.'

'This is ridiculous,' grumbled Dyson, but Barclay caught hold of him and pulled him to the ground.

It's worth a try,' he whispered. 'Lie still.'

Ben looked at the three men now lying motionless on the floor, their limbs spread, eyes closed behind the face vizors. 'Lovely!'

He walked towards the door, pulled back the opening lever and swung it open. His eyes met the blank stare of the Cyberman.

'You,' said Ben, pointing to him. 'Help us! Come in here quick. Something's happened to the others.' As he spoke, he sagged, grasped at the door frame, and staggered back into the room.

For a moment, the Cyberman paused suspiciously and looked through the open doorway. Then he caught sight of the prostrate, apparently dead, scientists. Ben slowly crumpled to his knees; his head bowed.

The Cyberman cautiously stepped inside: one pace; two paces. After three paces he stopped dead. Ben looked up, a whirring noise from inside the Cyberman's chest unit had begun; the lights on his frontunit were flashing wildly-like a pin-ball machine. The Cyberman stiffened, his hand opened; the Cyberweapon dropped.

Quick as a flash, Ben sprang up and grabbed the gun. The Cyberman was completely immobile; frozen as a lump of Polar ice. Ben pulled on the silver giant's arm, swung him around and, with one great shove, sent him crashing out of the room. He slammed the door, and threw the bolt. Behind him, the others started to rise.

'What on earth did you do that for?' said Dyson, getting to his feet. 'We could have escaped.'

'You're still not using your nut, chum. Escape! To where? We're O.K. right where we are.'

Barclay looked more hopeful. 'And they can't set off the bomb while we defend this room?'

The sailor nodded. 'Yeah, that's what I figure. All we've got to do now is sit tight and wait for Mondas to shrivel up like the Doctor said. We've got 'em.' For a moment he grinned at the two men triumphantly-then his face fell.

'But they've still got the Doctor and Polly!'

In the tracking room, Cyberleader Krang had just watched the tail end of the action in the radiation room. He had turned the monitor sets volume control up to catch Ben's last words.

The Doctor was standing, menaced by one of the Cyberguards. He looked over at Krang. 'There, gentlemen. Stalemate I would say, wouldn't you? Now perhaps we can talk!'

He placed his fingers together in a characteristic gesture.

The Cyberleader turned, and replied angrily. 'You forget-we can do what we like with all of you.' He indicated the technicians. 'And, of course, the girl.'

'Of course,' the Doctor nodded. 'But that won't save your planet, will it?'

Krang thought for a moment, then stepped forward and picked up the address mike. 'I will speak to them.' He looked across at the TV monitors, depressed a switch, and began speaking to the small figures of Barclay, Ben, Dyson, and Haynes on the screen.

'Listen to me. This close proximity of our two planets mean that one has to be eliminated for the safety of the other. The one to be destroyed will be Earth. We cannot allow Mondas to burn up. If you help us, we will take you back to Mondas with us. There you will be safe.'

'Oh yeah!' Ben shouted up towards the mike in the radiation room. 'For how long?'

'No,' Dyson whispered. 'Don't antagonise them. It could be our only hope.'

The watching Cybermen saw Ben push Dyson aside and look up directly at the monitors. 'The answer is no! We will just sit tight here until your planet breaks up. Now you'd better release the Doctor and Polly and send them down here. You'll need our help when Mondas is gone!'

The Cyberleader's voice began to speak with greater intensity. 'Mondas will not explode.' He turned to one of the other Cybermen. 'Take the old man out to the s.p.a.cecraft.'

'No,' pleaded the Doctor. 'I must stay here. You need me.'

'The Cybermen do not need anyone's help,' snapped Krang. He gestured and the Cybermen standing by the Doctor grasped his arm and led him from the room.

Krang turned back to the monitor screen. 'Now! We give you three minutes to start fusing the warhead. If you fail, you will never see your friends again!'

Dyson turned to the others. 'It's hopeless. We must do as they say.'

'It could be a bluff,' said Barclay uncertainly.

'Yes,' Ben agreed. 'Perhaps we should find out?'

Barclay shook his head. 'We must keep to our plan and sit tight. There are millions of lives at stake.'

'But Polly and the Doctor?' said Ben desperately. 'There must be something we can do!' He looked round and, before the others could stop him, rushed over to the TV monitor and ripped out the lead wires from the camera lens and microphone.

'What on earth did you do that for?' asked Dyson. 'Now they cannot communicate with us.'

'Yeah,' said Ben, turning back. 'They can't spy on us either, can they? I've got a plan...'

Aboard the Cyberman s.p.a.ceship, the Doctor was now seated beside Polly in another of the Cyberchairs. The Cyberguard was clamping the broad silver bands across his waist and arms.

'Doctor,' said Polly, 'can't you do anything?'

The Doctor shook his head and looked pointedly at the Cyberman. They waited until he had turned and left the room. 'At least, my dear,' replied the Doctor, 'they have allowed us some heat. They obviously mean to keep us alive.'

'But there's something else. A few minutes ago they started up some kind of engines.'

'Engines?' queried the Doctor.

'Yes. Listen!'

The Doctor became aware of the low throbbing vibration coming from the heart of the ship.

'It wasn't here before. They're not taking off, are they?'

'No.' The Doctor shook his head. 'Wait! Listen! Feel the vibration. I don't believe it is the engines.'

Their bodies were vibrating with the rest of the Cybership.

'Mondas must be causing this.'

'Mondas?' queried Polly.

'This s.p.a.ceship gets its energy from Mondas. It must be absorbing too much.'

'Do you mean it will blow up, Doctor?'

'I don't know, child. I really don't know...'

The men in the radiation room were having an urgent counsel of war. Ben had raised a bench on end to block the door observation window. For the first time since the advent of the Cybermen, the men felt that they were not being watched.

Ben pointed to the Z-Bomb. 'What's it weigh then?' Dyson smiled. 'You're not thinking of trying to carry that around, are you?'

'Who's asking you, laughing boy?' Ben retorted. He turned to Barclay. 'Can it be shifted?'

Barclay shook his head. 'It would be an impossible job, I'm afraid. To use it as you would intend to use it, that is.'

'Well,' Ben looked round, 'what is movable in this room? Something that a bloke could carry?'

'Nothing,' replied Dyson decisively. 'You're wasting your time and ours.' He looked at his watch. 'The three minutes is nearly up anyway.'