'He is with Bulic, helping to defend airlock five. I've decided to take your advice, Doctor, and call Sea Base Command for help!'

'Very wise,' said the Doctor drily. 'I hope it won't be too late.'

A frantic guard came running up to them. At the sight of the Commander he came to a sudden halt, giving a hasty salute. 'The monster's on its way, sir. It has already killed Doctor Solow. I found this by her body.'

Vorshak stared at the programme disc in astonishment.

The Doctor registered his reaction. 'Something wrong, Commander?'

'I hope not, Doctor. I'm not sure.' Vorshak slipped the disc into his pocket. 'All right, I'll deal with it. Make your way to airlock five and report to Lieutenant Bulic.

Lieutenant Preston, come with me to the Bridge.'

His communicator bleeped, and they all heard the panic in Bulic's voice. 'Commander Vorshak? They're in, sir.

The Sea Devils have broken through. They're everywhere.'

'Do your best to hold them, Bulic. I'm on my way to the Bridge to call Sea Base Command for help.'

The corridor by airlock five was filled with the sizzling of blaster-fire.

Bulic was shouting into the communicator. 'We'll try to hold them, Commander.'

'Do what you can. You've got to buy us some time.'

Bulic raised his voice. 'Back, all of you. Move back but as slowly as you can. We've got to delay them.'

Like the rest of the guards, Turlough fired and fell back, fired and fell back, retreating before the terrifying figures that stalked down the corridor. He took careful aim and shot the nearest Sea Devil squarely between the bulging eyes. With grim satisfaction, he saw it stagger and fall.

Fire.and fall back. Fire and fall back. Too busy to be frightened, Turlough fought the desperate rearguard action with the rest.

Nilson stood in the doorway of the computer bay, watching Maddox at work.

Maddox was a ghastly sight. Face sweating, eyes staring, he seemed on the point of collapse, yet he worked on with abrupt, galvanic movements, as if plugged in to some invisible power source.

Nilson studied him thoughtfully. 'Don't die on me, Maddox, not yet! Not till you have served your purpose.'

He took the control device from his pocket. 'Hurry, Maddox, there isn't much time!' He switched the control to a higher frequency.

Maddox began working even more quickly, his movements swift and jerky like someone in a speeded up film.

Nilson knew that Maddox could not last much longer at this pace. Soon he would simply burn out. But what did it matter, as long as his task was completed...

Icthar and his companions listened impa.s.sively to Sauvix's report. 'The ape-primitives are in full retreat. Most of the strategic areas are already under our control.'

'That is excellent news,' said Icthar gravely.

'Soon the Myrka will take the Bridge. When that is done, the outcome is certain.'

Vorshak strode onto the Bridge and tossed the programming disc onto the console in front of Nilson.

'Maddox's conditioning disc. You were ordered to guard it carefully, to return it to me when you had finished. Well?'

Nilson's astonishment was quite genuine. How on earth had Vorshak come by the disc? 'Doctor Solow had charge of the disc. I a.s.sumed she had already returned it to you.

Has something happened '

'Doctor Solow is dead, killed by the Myrka. Where's Maddox?'

'In the computer bay, checking on the synch circuitry.'

Vorshak turned to Preston. 'Get him in here!'

Lieutenant Preston went over to the door of the computer bay, opened it, and stood staring in horror at what she saw. 'Commander, come quickly!'

Vorshak ran to the door, and looked inside.

The place was a shambles, panels removed from the computer banks, whole areas of circuitry dismantled.

Maddox was still dismantling computer circuitry at the same incredible speed.

'Maddox,' roared Vorshak. 'What are you doing?' He dashed up to Maddox and tried to pull him away from the computer.

Vorshak was a big man and exceptionally strong.

Maddox was frail by comparison. Yet Maddox grabbed Vorshak and hurled him across the room like a child, then turned back to his work of destruction. Picking himself up, Vorshak returned to the attack. Once again he was thrown aside with the same careless ease.

Nilson appeared in the doorway and stood watching impa.s.sively. As Vorshak was about to attack for the third time, Nilson jabbed at the controls of the device hidden in his pocket.

Immediately Maddox went rigid, hands clawing at his temples. He swayed for a moment and then fell.

Preston knelt beside him feeling his pulse. 'He's still alive.'

Vorshak was staring grimly around the computer room.

'Never mind about Maddox. Check the damage to the computer!'

The Doctor and Tegan waited tensely beside the UV convertor. They could hear shouts and screams, the sizzle of blaster-fire and the angry roars of the Myrka. The sounds had been getting louder for some time, and now they were very close.

The Doctor looked at Tegan. 'Well, here it comes.

Pretty soon we'll know if my theory is correct!'

There was a deafening roar, and the Myrka appeared around the corner. It paused for a moment at the sight of the Doctor and Tegan. The terrifying dragon-like head swung round, and the red eyes glared threateningly at the Doctor.

Almost as if it recognised him, the Myrka give a challenging roar and advanced towards them.

9.

The Hostage Tegan cowered back as the monster advanced. 'Switch that thing on, Doctor. Switch it on now now!'

'Just a few more feet,' said the Doctor softly.

The Myrka came closer and closer, until it was almost upon them. Just as it seemed that the great tail must lash round and destroy them the Doctor threw the convertor switch.

'Cover your eyes, Tegan,' he shouted.

Suddenly the corridor lighting turned to a glaring white ultra-violet light.

The Myrka jerked to a halt. It writhed in agony for a moment and then toppled over sideways, landing with a thud that shook the corridor. It lay twitching, steam rising from its body. A final convulsive jerk, and it was still.

Cautiously Tegan moved closer. 'Is it dead?'

'Very,' said the Doctor with satisfaction. 'Let's get back to the Bridge.'

An instrument on Tarpok's belt gave an alarm signal. He took it out and studied it for a moment. 'The life-force of the Myrka does not register, Icthar. It has been destroyed.'

'The ape-primitives are more cunning than we had thought. Sauvix, divert your Warriors. They must capture the Bridge without delay.'

Sauvix saluted. 'At once, Icthar.'

Vorshak looked round the wrecked computer bay, down at the unconscious body of Maddox and then at Nilson. His voice was cold and furious. 'You were in charge of the Bridge, Nilson. How could you not know what was going on here?'

Nilson made no reply.

Lieutenant Preston opened a locker. She recoiled in horror at the huddled shape within. 'It's Karina, Commander. She's dead strangled...'

Vorshak's face hardened. 'I don't know what's been going on here, Nilson, though I shall find out. But you're responsible and, I promise you, you'll answer to a court martial. First you're going to revive Maddox and re-condition him. I want him ready for synch-up as soon as possible. I'm going to signal Sea Base Command.'

Nilson shook his head. 'I'm afraid I can't do that, Commander.'

'You'll do it all right ' Vorshak broke off. Nilson was covering him with a pocket-blaster.

'Your weapon please, Commander,' said Nilson calmly.

'Yours too, Lieutenant.' Nilson waved his blaster. 'Now, back to the Bridge.'

Helplessly they obeyed.

'What exactly do the Silurians want, Doctor?' asked Tegan.

'Oh, that's obvious, surely. Control a Base like this with its proton missiles and you can control the Earth.'

'Or destroy it?'

'Very probably.'

They carried on down the corridor, heading for the open door that led to the Bridge.

Nilson was herding Vorshak and Preston out of the computer bay at blaster-point.

'I trusted you, Nilson,' said Vorshak angrily.

'Don't take it too hard, Commander. Doctor Solow and I were only doing our duty as we saw it just as you are.'

The Doctor burst into the Bridge area, Tegan behind him. 'Commander, the Myrka has been destroyed ' He broke off noticing with some surprise that Nilson seemed to be covering his superior officer with a blaster. 'What's going on?'

'It seems that Nilson here is an enemy agent, Doctor,'

said Vorshak bitterly.

'Enemy agent? You mean he's working for the Silurians?'

'No. For the East Bloc.'

The Doctor said impatiently. 'Look, this is no time for your petty human feuds. Do you know what you're doing, Nilson?'

'I know very well, Doctor.'

'Do you?' snapped Vorshak. 'Do you really? Before long the Sea Devils will be in control of this Base.'

'Which means they'll have control of the proton missiles,' said the Doctor.

Nilson didn't look particularly worried. 'Missiles they cannot fire. Maddox rigged the computer under my instructions. The missiles are disarmed irreversibly.'