'So it is. Perhaps it's time for a little mutual trust.

Turlough was telling the truth, you know.' With that, the Doctor handed the blaster to the astonished Vorshak, and started climbing out of his disguise.

Bulic reached for his weapon, but Vorshak stopped him with an abrupt gesture.

There was a bleep from the intercom, and Vorshak crossed over to it. 'Vorshak here.'

Even over the intercom, Lieutenant Preston's voice sounded tense, uncertain. 'Commander, I think we've found the intruder's craft.'

'Go on,' snapped Vorshak.

'It's amazing. I don't think it can possibly come from this planet... it's... it's bigger on the inside than the outside.

There's a control room, corridors, more rooms, all packed into a little blue box.'

'Is it armed?'

'Not as far as we can tell, Commander.'

Vorshak stood for a moment, considering this astonis.h.i.+ng new information. 'Very well. Leave the craft alone. Put a guard on it, and report back to the Bridge.'

'Well, Commander?' said the Doctor gently.

Vorshak looked at Turlough. 'Multi-dimensional.'

That's what the boy had said. 'It seems your friend may have been telling the truth after all.'

Icthar stood waiting on the Bridge of the Silurian s.h.i.+p. He showed no tenseness, no excitement, even though the plan that might well determine the fate of all his people, and of the Earth itself, was about to be put into effect.

Sauvix approached him and saluted. 'All is ready. The Warriors are armed and at their battle stations.'

'Excellent, Sauvix.' Icthar turned to his two companions. 'Scibus, Tarpok, commence the power build-up. It is time to begin.'

Clawed hands moved over the strange organic-looking controls and soon the Silurian s.h.i.+p began throbbing with energy, like a living being. It rose from the undersea volcano crater and slid through the murky waters, like some great predator, poised for attack...

Tegan was growing impatient in her'hiding place. The Doctor had been on the Bridge for some little time now.

Since he hadn't returned with Turlough, something had presumably gone wrong. In which case, it was up to Tegan to rescue them both.

She stepped cautiously out of her hiding place, and found herself facing a young woman and a guard, both armed.

The guard covered her with his blaster-rifle.

The young woman said politely, 'Did you want something?'

'All right, Doctor,' said Commander Vorshak. 'I'm prepared to believe that you're not actively hostile, despite your interference with the reactor.'

The Doctor looked rather shamefaced. 'Thank you.

You're very kind.'

'But that doesn't mean I trust you completely. I should like to see a demonstration of this s.h.i.+p of yours.'

'Well, when I've repaired her, you'd be most...'

He broke off as Tegan came onto the Bridge, escorted by a woman officer and a guard.

Lieutenant Preston saluted. 'The third intruder, Commander. I found her lurking outside.'

Turlough moved to join his fellow captive. 'Are you all right?'

Tegan nodded, looking worriedly at the hostile group around them.

Suddenly the wail of an alarm filled the Bridge.

Lieutenant Karina called, 'Commander, the screen!' A cylindrical shape was registering on the main scanner screen. 'It's some kind of s.h.i.+p, Commander. And it's heading straight towards the Base.'

Vorshak studied the image. 'Isn't that the thing we picked up before?'

'I think it is, sir.'

Maddox called across from his console. 'We're getting a clear sensor-reading, Commander. It's a s.h.i.+p all right. But it's not one of ours. The readings don't correspond with any known enemy s.h.i.+p either.'

A terrible recognition was dawning in the Doctor's mind.

The shape on the screen continued its remorseless advance.

'Prepare to fire energy-tracers,' ordered Vorshak.

'No!' shouted the Doctor. 'You mustn't. I think I know what it is!'

Vorshak swung round. 'Are you saying you recognise that thing?'

'Yes. You mustn't attack it.'

'You're hardly in a position to give orders here, Doctor.'

'Ready to fire, Commander,' reported Bulic.

'If you open fire you'll regret it,' warned the Doctor. 'I tell you, I know what that s.h.i.+p is!'

'Are you telling us not to defend ourselves?'

'I'm telling you you have no defence. That's a Silurian battle cruiser.'

Vorshak stared at him. 'Silurian?'

'The race that ruled Earth before your species evolved.'

Hands poised over his weapons console, Bulic looked at Vorshak. 'Do we fire, sir?'

'Trust me, Commander,' urged the Doctor. 'Make contact with them, find out what they want.'

'It's what we want that matters, Doctor and that's to keep them away from this Base. Bulic, open fire!' Bulic's hands stabbed at the console. An energy-beam sped towards the intruder.

Suddenly the screen flared white with a blinding flash.

On the Bridge of the Silurian battle cruiser a gauge glowed steadily.

'The deflector is locked onto their energy-beam.'

reported Scibus emotionlessly. 'Their external weapon system is now suppressed.'

Icthar's voice was equally calm. 'Proceed as planned.'

Monitor screens on the Bridge showed only a blur of static.

'Fire again!' ordered Vorshak.

Karina looked up from her console. 'The weapon systems are dead, sir.'

Vorshak glared suspiciously at the Doctor. 'You knew that would happen!'

'I did try to warn you.'

'What have they done?' asked Preston.

'It's a particle suppressor they turned your own energy-beam back on to you. They could have blown this base apart if they'd wanted to you certainly gave them enough reason.'

The Silurian s.h.i.+p glided through the depths and settled gently onto the ocean floor close to the Sea Base.

On the Bridge, Scibus reported, 'We are in position.'

'Release the Myrka,' ordered Icthar. 'Sauvix, proceed to your station. When the Myrka begins its work, you and your Warriors will commence the attack on the main entry point.'

Vorshak was desperately trying to get his demoralised Bridge crew back into action. 'I want full damage reports from everyone as soon as possible. Maddox, set the computer scan to a.n.a.lyse the Silurian weapon. Bulic, all guards to battle stations.'

For a moment the Doctor and his companions were forgotten in the bustle of activity.

Tegan said, 'I gather you've met these Silurians before, Doctor? When?'

'A long time ago,' said the Doctor sombrely. 'I let them down before now it seems I'll do so again.' The Doctor was silent for a moment, remembering his previous encounters with the extraordinary reptilian species. The Silurians had evolved millions of years before man. They had developed a technologically advanced civilisation with an emphasis on bioengineering. In addition, many of them had developed almost mystic powers.

This reptile civilisation had ruled the Earth until their astronomers had warned them that the globe was about to be struck by a rogue planet from outer s.p.a.ce. To escape the catastrophe, the Silurians had put themselves into deep hibernation, sealing themselves in shelters deep beneath the ground or the seas.

The catastrophe the Silurians feared had never happened. Instead of striking the Earth the little planet had gone into orbit around it, becoming Earth's moon.

The reactivating triggers had failed to work, and the Silurians had slept on, until...

It had all happened during his third incarnation, the Doctor recalled, the time of his exile on Earth. The energy of an underground research centre had revived a Silurian colony in a nearby cave system.

The great reptiles had awakened only to find that Earth, their Earth, had been over-run by the apes. In the inevitable conflict that followed, the Doctor had tried to act as peacemaker. For a time it had seemed that he might succeed in convincing Silurians and Humans alike that the Earth was big enough for both of them. But in the end the extremists on both sides had triumphed. The Silurians had been entombed in their underground base.

Events had followed the same tragic pattern when the Silurians' marine cousins, the Sea Devils, had revived.

This time the situation had been worsened by the meddling activities of the Doctor's old enemy, the Master.

The Doctor had been forced to choose the lesser of two evils, thereby bringing about the destruction of the Sea Devils.

Turlough's voice interrupted the Doctor's reverie.

'These Silurians, Doctor are they hostile?'

The Doctor nodded. 'They are now. But they're honourable too, in their own way. All they want is to live in peace, on a planet they consider their own.'

Silurians and humans two warring species, thought the Doctor, so different, and yet in some ways so alike: both intelligent, aggressive, arrogant, both convinced that the Earth was theirs by right. Was there really any chance at all of making peace between them? Very little, thought the Doctor sadly. But all the same he knew he had to try.

He moved closer to Vorshak. 'Commander!'

'Yes, what is it?'

'You can't fight them, you know,' said the Doctor quietly. 'Certainly not alone.'

'I can try.'

'Inform your people on the surface. Tell them what's happening down here.'

'I can't. Things have reached crisis point up there. I've been ordered to maintain radio silence. I can't risk revealing Sea Base Four's position to the enemy.'

'East or West, friend or enemy the distinction will be lost on the Silurians, I a.s.sure you,' said the Doctor grimly.

'To them you're all the same. Ape-descended primitives.

An evolutionary error they're determined to correct.'

The intercom bleeped and an anguished voice shouted.

'Commander, there's something outside airlock one. It's trying to force the outer door!'

Vorshak swung round on Bulic. 'Get a combat team down there. Lieutenant Preston, deploy the duty guards to cover the other airlocks.'