The young girls's clothes hung limply round the bodies of the shrinking hags. Older and older grew the two companions as the TARDIS travelled through time and s.p.a.ce. Soon their flesh would be dust.

'Like Mawdryn in the lab,' whispered the Brigadier, peering aghast at Tegan and Nyssa's withering bodies.

'Mawdryn!' cried the Doctor. 'They've been contaminated...' He had only the merest intuition of the terrible syndrome from which, within minutes, both girls would surely be dead. He wracked his brains for some quick antidote. 'The transfiguration can be contained,' he muttered, desperately near panic.

'Stop!' Nyssa's strangled cry was barely audible, but the Doctor immediately leaped to the console.

'Stop! That's it!' He instantly reversed the co-ordinates.

'Travelling through time has accelerated the degeneration.'

The Brigadier looked over the Doctor's shoulder at the flashing lights on the console. 'You've stopped the TARDIS?'

'More than that.' The Doctor stared anxiously at the mummified faces of Tegan and Nyssa. 'We're going back to where we started. I just hope it induces a proportional remission.'

The younger Brigadier's knuckles were raw with banging against the walls of his prison. He had explored every inch of the sealed chamber and attacked the surround of the door with penknife, pipe-cleaners and ballpoint pen, but to no avail. If ever he caught up with that impudent whippersnapper, Turlough...

He found himself staring at the ornamentation around the door. Part of the frieze seemed to be loose. He ran his hand gently over the entablature; there was a click, and the door swung back. He was free.

Weak with relief the Doctor knelt over the two exhausted girls.

'It worked!' observed the Brigadier gruffly, equally gratified to see Tegan and Nyssa returned to their normal selves.

'Doctor, what went wrong?'

The Doctor tried to describe the infection they must have picked up when they carried Mawdryn into the TARDIS; a viral side-effect of the mutants' constant experimentation. The Brigadier wondered, ominously, whether he too would succ.u.mb to his brief contact with the creature in the laboratory.

'So we can't travel through time?' said Nyssa, as she realised the implications of what the Doctor had just told them.

'We don't need to time-travel,' interrupted Tegan, who only wanted to get back to Earth.

The Doctor shook his head. 'I have to programme a temporal deviation to escape the warp ellipse.'

'Look!' The Brigadier pointed at the scanner. Standing, like a guard of honour, outside the TARDIS, dressed in their finest robes, were Mawdryn and his brothers in exile.

'They knew this was going to happen.'

'That's why they let us go so easily,' said the Doctor bitterly.

'You mean we're stuck on this ship?'

'I wonder!' The Doctor returned defiantly to the console. 'If I reversed the trajectory...'

'The Doctor will not give up so easily,' said Mawdryn to his comrades, as the TARDIS dematerialised a second time. The confident smile disappeared from his face as a middle-aged Earthman in a blue blazer rushed into the empty s.p.a.ce left by the police box.

It had never occured to the younger Lethbridge-Stewart, when he left to reconnoitre the ship, that the time-machine could leave without him, and it had been a considerable shock as he turned the corner by the staircase, to see the light on the police box already flashing. He sprinted forward... but too late.

The presence of the alien from the TARDIS, together with seven more of similar ilk was a further surprise to the Brigadier. But it was nothing to the confusion and dismay of the eight vigilants at his own arrival.

'Brigadier!' exclaimed Mawdryn, who had just seen the same military gentleman leave with the Doctor.

'This man is also in the TARDIS,' warned a fellow Mutant.

'He is a deviant!' cried another.

'There has been temporal duplication!'

There was consternation amongst the mutants.

'The TARDIS will soon return. The imbalance could be cataclysmic,' declared Mawdryn. 'For your own safety you must return to Earth at once.' He grabbed the Brigadier by the arm and hurried him in the direction of the control centre.

'So far so good.' The older version of Brigadier Lethbridge-Stewart was anxiously watching Tegan and Nyssa as they time-travelled away from the ship. The Doctor stood beside the console, hand poised over the controls.

'It's no good!' wailed Nyssa in a plaintive voice.

'But nothing's happening,' protested the Brigadier.

'Oh yes it is,' said the Doctor in despair.

Lethbridge-Stewart looked more closely at the two girls.

There was a look of bland innocence on Nyssa's face, a softening of the aggressive line of Tegan's jaw. They were both suddenly thinner, shorter...

'We're travelling in the opposite direction,' explained the Doctor. 'It's having the reverse effect.'

'Stop! Stop!' piped the voices of two tiny children.

As Tegan and Nyssa regressed towards infancy, the Doctor reversed the direction of the TARDIS.

Mawdryn returned from the control centre in time to see the police box rematerialise at the foot of the stairs.

Everything was happening as he had predicted. All things proceeded towards the ending.

Leaving the Brigadier to comfort his two companions, the Doctor returned to the console where an intermittent buzzing had begun to sound in the communications section. Someone must be trying to operate the transmat capsule. 'Obviously Turlough taking your other half to the centre of the TARDIS.' He explained his plan for avoiding the Blinovitch Limitation Effect to the older Brigadier.

'Can the capsule do that?'

'Only when the TARDIS is clear of the ship. Until that happens the transmat can't take place. The capsule will return to its terminal.'

The junior Brigadier opened the door of the silver sphere into which he had been so unceremoniously bundled. He was still on board the alien's ship. Lethbridge-Stewart was not surprised; he had never really believed the creature when he pretended to be the Doctor, and he certainly wasn't going to be persuaded that this bauble would transmit him to Earth.

There was a sudden bleeping, quite different from the whirring and buzzing when he operated the so-called transmat control. He caught sight of a rather familiar round object wired into the control panel. He could swear that was the Doctor's homing device. But how... As the Brigadier's hand went to his blazer pocket, it froze as if paralysed by an electric shock that deuced static again.

He looked at the globe in front of him and smiled. That was the homing device all right indicating the presence of the TARDIS. The alien could keep his transmat capsule.

This one was going home by police box.

Turlough returned reluctantly to the dormition chamber.

He had no reason to believe that the younger Brigadier would prove any less blisteringly choleric at his incarceration than would have been the older, and more familiar schoolmaster. But he needed to follow the Doctor's instructions, if only to guarantee his escape from the ship. At least, with the 1983 Lethbridge-Stewart safely on his way back to Earth, the release of the prisoner behind the icon could hardly upset the Black Guardian. Not that he particularly cared; the owner of the TARDIS would appear to have got the better of the man in black.

As he approached the inner door he saw the open door.

An ominous red glow filled the chamber. Turlough began to shiver.

'You have failed me!' The voice of the Black Guardian reverberated angrily in the empty room.

'No!' Turlough trembled in the doorway.

'The Brigadier is free.'

'I'm sorry.'

'So near the annihilation of the Doctor, and you risk all with your negligence and stupidity.'

So the Doctor had not escaped from the ship at all. 'I can still keep the two Lethbridge-Stewarts apart,' pleaded the boy.

When the unseen voice sounded again, it was darker and more terrible than ever before. 'If you fail me again, I shall destroy you, Turlough!'

The Doctor leaned despondently over the console. There was nothing he could do to clear the ship without hurting the two girls.

'What are we going to do?' asked Nyssa.

The Doctor was silent.

'We can't stay in the TARDIS for ever.'

They all looked up at the scanner with its view of the reception committee outside.

'Well, Doctor?' said the Brigadier.

Still without saying a word, the Doctor opened the main doors and walked out of the control room.

The Time Lord stood aloof from the rest of them, his head slightly bowed. It was Nyssa who confronted Mawdryn.

'You knew that would happen!'

'Yes, Nyssa.' Mawdryn spoke with unexpected tenderness. 'But there was no conspiracy to harm you.'

'What happens now?' asked Tegan.

'You will remain on the ship.'

Tegan was stunned. 'For the rest of our lives?'

'You are fortunate,' said Mawdryn sadly. 'Your journey will be short. Ours is without end.'

Nyssa and Tegan looked disbelievingly at each other, then turned to the Doctor.

The Doctor said nothing.

Brigadier Lethbridge-Stewart took a step towards the mutants. 'We are not leaving those two girls on your ship.'

'Take them with you in the TARDIS,' replied Mawdryn, 'and they will die.'

'Are you telling me that with all those facilities you can't come up with some sort of antidote?'

'We have no restorative for Tegan and Nyssa.' 'Doctor, you must have some ideas?'

The Doctor said nothing.

Lethbridge-Stewart turned back to Mawdryn. 'When we were in the laboratory you claimed the Doctor could help you through that machinery.'

'Yes, but only of his own free will.'

'Then he can do the same for Tegan and Nyssa?'

'That is a question you must ask the Doctor.'

'Well, Doctor?' said the Brigadier.

The Doctor said nothing.

The eight mutants stared at the Time Lord.

'Doctor!' pleaded Tegan.

'Doctor!' begged Nyssa.

'Take me to your laboratory,' said the Doctor to Mawdryn.

The procession advanced slowly along the corridor.

First, seven mutants in their finery; then Mawdryn and the Brigadier an odd partner in his hacking jacket and cavalry twills; then the girls; and finally the Doctor, proud and silent, like a condemned man determined to die with dignity.

Mawdryn spoke the Doctor's epitaph as he walked with Brigadier Lethbridge-Stewart. 'The Doctor is a Time Lord, but he chose to involve himself; soon he will be a Time Lord no longer. That is his reward for compa.s.sion.'

Just as he had thought - the TARDIS had come back to the ship! The Brigadier from 1977 hurried into the control room. It was deserted. What were those two young women up to now!

Turlough had seen the procession enter the laboratory.