Mawdryn's putrid flesh quaked.

'Perhaps it was the Doctor,' added Tegan.

' I I am the Doctor!' shrieked Mawdryn. 'Dematerialise immediately!' am the Doctor!' shrieked Mawdryn. 'Dematerialise immediately!'

Nyssa hesitated.

'Time is running out. We must leave this place at once.'

The gentle Nyssa could no longer endure the distress of a man who might be the Doctor. Her hand moved to the lever that would activate the TARDIS.

'No!' Tegan dragged Nyssa away from the console, but already the slow rise and fall had begun.

The journey to the ship did not take long; within minutes of real time, they had entered the warp ellipse and the old police box made a second incongruous appearance inside the sombre vessel.

Nyssa opened the scanner.

'The ship!' cried Mawdryn.

'Is it indeed,' muttered the Brigadier suspiciously. The marble hall he could see on the scanner was not his idea of the inside of a s.p.a.cecraft.

Mawdryn prepared to leave. 'You will stay in the TARDIS,' he informed the Brigadier and the girls.

Tegan quickly placed herself between Mawdryn and the double doors. 'If you're in a regeneration crisis you'll need all the help you can get.'

'No!' He was surprised at her continuing defiance.

'She's right, Doctor,' said the Brigadier, trying at the same time to show respect for a possible Time Lord, yet still support the plucky Australian.

'I must go into the ship alone.'

Tegan stood her ground. She was not going to let the creature from the transmat capsule out of her sight.

Mawdryn staggered giddily. The atmosphere of the TARDIS had helped restore his strength, but his conflict with the Earthchild had dissipated that new-found energy. In a weary, broken voice he began to plead with her. 'You do not understand the nature of the transmogrification. The unique restorative conditions of that vessel' he indicated the screen, then turned to his three fellow pa.s.sengers - 'the presence of other life-forms would inhibit the reparation.'

The Brigadier and Nyssa glanced uncertainly at each other, while Tegan glared implacably at Mawdryn. 'We've all seen the Doctor regenerate before and the evidence suggests that without the presence of other life-forms he could die.'

Mawdryn abandoned reasoned argument. 'Open the doors,' he screamed.

Nyssa moved to the console, but Tegan pulled her out of the way. 'You're not going out into the ship!'

Mawdryn began to groan with pain and rage.

'Either you stay here or we go with you.'

The creature was fighting for air. He clawed and twisted like a drowning cat. Tegan willed herself not to relent, but Nyssa was less resilient. She edged towards the door control lever.

'Nyssa! He could reactivate the beam and the TARDIS would be trapped on the ship for ever.'

'But if he is the Doctor...'

'I don't believe it.'

'The doors!' howled Mawdryn.

'No!'

'You are destroying me!' He collapsed exhausted on the floor and began to sob. 'Spare me the endurance of endless time, the torture of perpetuity.' He was now at their mercy.

'For pity's sake, release me!' he begged.

Tegan could hold out no longer. She turned to the Brigadier. The Brigadier was a professional fighter, but even he could not willingly inflict suffering on a defenceless man. 'Let him go,' he ordered.

Nyssa opened the doors and Mawdryn stumbled out of the TARDIS.

Tegan was trembling from the strain of the confrontation. 'I hope you know what you're doing,' she said quietly.

'Keeping him in the TARDIS might have killed him,'

answered the Brigadier.

'And we can't be certain he isn't the Doctor.'

'Can't we?'

They all looked at the scanner to see Mawdryn drag himself from the time machine and merge into the shadows of the alien ship.

The Doctor stood beside the obelisk staring disconsolately down into the valley. Earth was certainly an attractive planet - his favourite in fact - but there is nowhere in the Universe, however beautiful, that does not lose its charms when it becomes a place of exile. 'If only I had some sort of equipment for tracking the TARDIS,' he muttered to himself.

'What you need is a homing device.'

The Doctor smiled sardonically at his old friend.

'Thank you, Brigadier. I forgot you had such a remarkable talent for perceiving the obvious.'

'I have such an object,' observed the Brigadier, as inconsequentially as if he referred to a boy-scout's pocket compa.s.s.

'What did you say!'

'Tegan gave it to me.'

'Brigadier that was six years ago. Is it too much to hope that...?'

The Brigadier remained infuriatingly unaffected by the Doctor's excitement. 'Never know when something's going to come in useful.'

'You mean...'

'No idea what it was or where it came from.' For the Brigadier, the sudden remembering merely provided an explanation for the gla.s.s ball at the bottom of his junk box.

For the Doctor it was the only chance of making contact with his TARDIS. 'Brigadier, where is it?' he shouted, losing all patience.

'Back in the hut.'

The Doctor was already off down the hill. 'Hurry, man.

We haven't a moment to lose!'

There were bits of used string, pitted with sealing wax, carefully unravelled and rewound; a broken alarm clock; a b.u.t.ton stick; an old penknife; some clothes coupons; a twisted tube of moustache wax; a gas mask; a pair of nut-crackers; a patent self-stropping razor... and there at the very bottom of a perfectly useless collection of bits and bobs in the rusty ammunition box: the Doctor's homing device.

The Doctor held it lovingly in his hands. As he made contact with the activator the plaintive bleep brought a boyish grin to his face.

It was a puzzle to the Brigadier how he could so completely have forgotten that on Jubilee Day the same little box of tricks had directed Tegan to the TARDIS on top of the hill.

Once more the sensor was indicating the location of the Doctor's time-machine.

'I should have known.' The Doctor had stopped smiling.

'You've located the TARDIS?' Till then Turlough had been silent.

'It's gone back to the ship.'

The Doctor now had a far clearer picture than the Brigadier of what had happened on 7 June 1977. The wounded creature from the sphere, unable to endure a second journey in the capsule, had needed the TARDIS to return to his ship. Tegan and Nyssa must have gone with him, believing him to be himself the Doctor.

The Brigadier was equally grim-faced as the Doctor explained the predicament. 'So Tegan and her friend are marooned in s.p.a.ce at the mercy of this thing thing.'

The Doctor wondered fearfully what sort of creature his impersonator could be, who voyaged for so long in such a strange ship.

'Travelling in a warp ellipse', he explained to the Brigadier and Turlough, 'is a form of infinity.'

'You make him sound like some kind of Flying Dutchman.'

The Doctor stared at the Brigadier. 'Condemned to sail the Universe for all eternity?' It was an interesting idea.

'Nonsense. No one is immortal...' The Brigadier shivered. 'Are they?'

But the Doctor had already turned his attention to getting on board the alien's s.p.a.cecraft.

'You can't use the capsule!' protested Turlough.

'There's no beam.' If he felt any guilt about the damage to the cylinder, he concealed it from the Doctor.

'You're forgetting this.' The Doctor held up his homing device. 'The TARDIS is on board the ship, and this will home in on the TARDIS.'

The Brigadier groaned. Back to the obelisk again!

Brigadier Lethbridge-Stewart was the last to reach the top of the hill. Helped by Turlough, the Doctor was already installing the homing device in the navigational unit of the transmat capsule as his exhausted friend stumbled breathlessly through the door of the silver sphere. 'This is the third time today I have yomped up this wretched hill!'

he grumbled.

The Doctor finished his work. 'Good of you to see me off, Brigadier.'

But Lethbridge-Stewart had no intention of letting the Doctor out of his sight. Heaven knows what trouble the man would get into, left to his own devices.

For reasons far more devious than those of the Brigadier, Turlough was determined to stick with the Doctor as well.

'Don't be ridiculous!' protested the Brigadier, who had had quite enough of the precocious young man for one day.

'The Doctor needs my help.'

The Brigadier grunted. There was no denying it, the boy had remarkable skills. He wondered what Mr Sellick was going to say about it all.

The Doctor closed the capsule door.

'How long will the journey take?' The Brigadier braced himself, expecting at any moment to be blasted off the hill like a cannon ball.

The Doctor opened the door again.

The old soldier blinked as he looked out into the control centre of Mawdryn's ship.

As Mawdryn left the unique atmosphere of the TARDIS he felt the strength go out of him. He clawed at the hard, smooth walls of his ship, but there was no resilience to his limbs. He grasped at a pillar, but on contact with the faceted marble his flesh crumbled like fly-blown fungus.

He collapsed slowly to the floor. The Doctor's red coat was already sodden with pus and liquified flesh. Mawdryn moaned at the pain of his dissolution and longed for oblivion. But now he needed the laboratory. He must reach the apparatus. He must go on.

He undulated what remained of his body, and his viscous torso slid slowly forward along the corridor. Every inch of the way was the most appalling agony. He began to fear that time was running out. The girl, Tegan, might take the TARDIS back to Earth, to be reunited with the genuine Time Lord. Never again would there be the chance of an ending. Mawdryn needed help. He turned off the main companionway,.

The unearthly faces gazed haughtily down as Mawdryn slithered into the hall of likenesses. He stopped below the central icon that had so disturbed the Doctor on his first exploration of the ship. With a supreme effort of will he raised himself up towards the frame. 'Mawdryn has returned!' he cried. 'It is time for the awakening. Help me!'

But neither help nor answer came.

Fearing that his frail voice had not reached his comrades in the dormition chamber, Mawdryn called more loudly. 'My brothers, awake. Mawdryn has returned. I have brought to our ship a TARDIS...'

He felt himself weakening. Hope alone gave him the strength to continue. 'The time of our ending is near!' he called.

But the news fell upon deaf ears.

'Help me!' Mawdryn strained again to reach the chamber release, but the effort was beyond him, and with a cry of despair he fell back to the floor.

In the TARDIS control room, the Brigadier, Tegan and Nyssa stared at the screen. There was no sign of any activity in the ship. The Brigadier began to suspect that Tegan had been right about the 'Doctor'. But he could never have imagined that the greatest danger, out there in the ship, was himself; for it would have been beyond the comprehension of the common-sense military man in the control room that he could ever meet up with his own person - some six years older.

'Right!' The Brigadier turned to the doors. 'Time for a recce. I think we should keep an eye on this character.'

'I'm coming with you.'

'You girls are staying here.'