'It is you ape-primitives who are mad,' said Icthar. 'You have developed this weaponry. We cannot be held responsible for it.'

'Contact the heads of governments,' urged Vorshak.

'Make your demands, tell them you want a share of the Earth. They'll listen.'

'Your race has had its chance,' said Icthar implacably.

'Please, try one more time, for pity's sake.'

'It is too late for pity,' said Icthar. 'It is much too late.'

In the chemical store the pump was humming steadily, feeding Hexachromite gas into the ventilation system.

The Doctor said thoughtfully, 'Of course, it will take some time for the gas to spread.'

'Will it work fast enough?' asked Tegan anxiously. 'How long before it floods the whole Base?'

'I rather hope that won't be necessary. If I can get back to the Bridge, I may still be able to reason with Icthar, persuade him to abandon the missile launch.'

Turlough looked at him in amazement. 'You're mad, Doctor. You've tried that once already, remember?'

'Ah, but this time I'll have a counter-threat to back up my arguments,' said the Doctor, hopeful as ever.

Icthar studied the flow of data across the screen. He looked almost pityingly at Vorshak. 'Soon it will be all over.'

Vorshak stared at him in silent anguish, poised to make a last-minute attack.

Suddenly Tarpok called, 'There is computer resistance to the Manipulator.'

Hope flared up in Vorshak. 'I told you you'd never succeed!'

'Increase the power, Tarpok,' said Icthar placidly. 'It seems that your computers are as stubborn as you humans, Commander. But do not be deceived. We shall overcome you both in the end.'

The Doctor stood watching sadly as the pump drove the deadly gas into the ventilation system. 'Right, that's it, I want the rest of you to go back to the TARDIS and wait for me. You left the door ajar, Lieutenant Preston?'

She nodded.

The Doctor went on, 'You'll be safe inside the TARDIS whatever happens.'

The siren rang out again, this time with a different note.

'They've changed to yellow alert,' said Bulic.

The Doctor nodded. 'We must go.' He looked round the little group. 'Good luck, all of you. I'll join you in the TARDIS as soon as I can. If I don't Tegan and Turlough will have to do the best they can.'

They heard a voice behind them. 'So Doctor, I have found you!'

They turned.

Sauvix, leader of the Sea Devils was standing in the doorway.

12.

Sacrifice On the Bridge all eyes were fixed on the read-out screen, on the flas.h.i.+ng words 'YELLOW ALERT'. Incongruously the Intercom beeped.

Icthar touched a control. 'Speak!'

They heard Sauvix's voice. 'I have the Doctor, Icthar.'

'And you have your orders,' said Icthar implacably. Kill him.' He turned back to the screen.

'Sauvix, you must listen to me,' pleaded the Doctor.

There was a gleam of what might have been amus.e.m.e.nt in the great bulbous eyes. 'No, Doctor. You must die. But first, switch off that pump.'

As the Doctor moved towards the pump, Lieutenant Preston looked swiftly around her.

Turlough had laid his blaster-rifle on top of one of the cylinders. It wasn't all that far from her...

As Sauvix's weapon moved to cover the Doctor, she made a desperate lunge for the blaster. She s.n.a.t.c.hed it up, aimed...

Sauvix whirled round and shot her down.

As Preston died, her hand tightened on the trigger of the blaster. It fired, burning a hole in a cylinder close to Sauvix's head.

The gas spurted out. Sauvix reeled, clutching his gills, and fell dying to the floor.

Tegan was kneeling by Lieutenant Preston's body.

'She's dead, Doctor.'

The Doctor nodded sadly. 'Such a waste.'

'She saved your life,' said Tegan. 'Don't let her sacrifice it for nothing.'

'I won't. I must get to the Bridge!'

'You'll need some help. We'll come with you, won't we Turlough?'

Turlough nodded resignedly. As far as he could see, anywhere on the Base was just about as dangerous as anywhere else.

'All right,' said the Doctor. 'Bring some of those oxygen packs. We may need them.' He pointed to a wall-rack which held oxygen cylinders with attached face-masks presumably some kind of rescue kit.

Turlough and Tegan took an oxygen pack each.

Bulic said, 'I'll stay here and keep things running. Good luck, Doctor.'

They hurried away.

The computer has stabilised,' said Scibus.

Tarpok said, 'Confirmed. The Manipulator has regained control.'

'You see, Commander?' said Icthar triumphantly. 'Now do you believe me? Silurian technology is invincible.'

Despairingly, Vorshak turned away.

The Doctor, Tegan and Turlough turned a corner just in time to see two Sea Devil guards stagger helplessly and collapse. Just above their heads, a cloud of gas drifted mistily from the ventilation grille.

'It's working!' said Turlough.

The Doctor frowned. 'It's working far too quickly. I can't bargain with Icthar if all his guards are dead.'

If all his guards are dead, we won't need to bargain, thought Turlough. Keeping the thought to himself, he followed Tegan and the Doctor down the corridor.

The countdown to missile launch had reached its final phase. All that was needed now was the final hand-print of the base Commander.

'Fetch him,' ordered Icthar.

Two Sea Devil guards seized Vorshak and dragged him towards the command console.

'No!' shouted Vorshak. 'No! I will not be responsible for the destruction of my own kind.'

But there was nothing he could do. For all his size and strength, Vorshak was like a child in the hands of the Sea Devils. They forced his hand into the hand-scan recess.

'The final phase,' said Icthar softly. 'It is almost done.'

Bulic checked the gauge on the gas cylinder. It was almost empty. Swiftly he uncoupled the cylinder, rolled it away, heaved a full one into place and connected it up.

Bulic had little faith in the Doctor's peacemaking efforts, and little interest in their success. As far as he was concerned the Hexachromite gas would deal with the Silurians very nicely.

The words 'RED ALERT' flashed on the screen.

Icthar moved slowly over to the command console. He reached out for the lever that would initiate the missile launch. Like his fellow Silurians, he was too absorbed to notice the gas-mist seeping from the ventilation grilles above his head.

He paused, savouring the moment and the Doctor and his companion dashed onto the Bridge.

Icthar was about to order the guards to fire, when quite suddenly he changed his mind. 'Disarm them,' he ordered.

A Sea Devil s.n.a.t.c.hed the blaster from Turlough's hand: 'Welcome, Doctor,' said Icthar. 'You are just in time to witness the missile launch.'

'Wait,' said the Doctor. 'You have been defeated, Icthar.

Your warriors are dying all over this Base. Abandon the Base and save yourselves.'

Icthar stared unbelievingly at him. 'The Silurians defeated? Are you mad?'

Tegan pointed to the cloud of gas drifting from the ventilation grille. 'Look at that it's Hexachromite gas.'

'Abandon the launch,' urged the Doctor. 'Leave the Base now, while there's still time to save your own lives.'

'It is unimportant that we die,' said Icthar hoa.r.s.ely.

'There are millions more in hibernation, ready to replace us.' 'And who will replace you, Icthar? With you dies the last of the Triad, custodians of the ideals of your race.

What will become of your people then?'

'You talk in vain, Doctor,' roared Icthar. He turned to his guards. 'Kill them. Kill them now!'

The Sea Devils raised their weapons, but their movements were slow and clumsy. They staggered helplessly for a moment, and then one by one they fell.

The Doctor moved to the intercom and flicked the switch. 'Bulic, can you hear me? This is the Doctor. It's over, Bulic. Turn off the gas.'

Icthar stared wildly at him, scarcely realising what was happening. He caught a whiff of the drifting gas, staggered and then recovered himself. 'Scibus! Begin ignition,' he ordered.

Scibus too was affected by the gas. With a last desperate effort he threw the lever. 'Missiles set,' gasped Scibus, and then crashed to the ground.

Icthar staggered and fell.

The Doctor ran to the Manipulator which was pulsing steadily. Tarpok lunged at him, but the movement was slow and clumsy. The Doctor dodged round him with ease, and Tarpok too collapsed.

The Doctor bent over the console. 'The missiles are set to fire, Vorshak. How long have we got?'

Vorshak looked at the digital countdown clock. It stood in 179. 'Less than three minutes.'

'What's the abort procedure?'

'A phased electrical charge aimed directly at the ignition circuit. It restores the launch to a simulation.'

'Then do it right away!'

'Impossible, Doctor. Only a trained synch operator can do it, and Maddox is dead.'

The Doctor was already heading for the synch-op chair.

'All right then, I'll have to do it myself.'