The Brigadier was not in the best of humour as he strode back to the school.

'Sir!' A plaintif cry came from across the meadow. 'Sir!

Sir!' It sounded like a small animal in distress.

The Brigadier turned. It was in fact quite a large animal that came tearing towards him over the gra.s.s. The Brigadier had never seen Ibbotson run before.

The boy staggered to a halt drawing in great lungfuls of air. 'It's Turlough, sir!' He swayed dizzily. The Brigadier grabbed him by the shoulders.

'We were on the hill, sir...'

'What?' snapped the Brigadier. Turlough had no right to have left the sick bay. He would be for the high jump this time.

'There was this great silver ball!'

The Brigadier snorted.

'Turlough went inside and disappeared.'

Brigadier Lethbridge-Stewart took a deep breath, about to explain that he had not just arrived on a banana boat, when he saw the tears in Ibbotson's eyes. The boy was shaking like a leaf. Perhaps there had been a genuine accident.

It was tough going, climbing to the obelisk, and the Brigadier was glad to rest for a moment while a flagging Ibbotson caught up. 'If you took more exercise,' he bellowed at the boy, trying to conceal his own puffing and blowing, 'not only would your body be less disgusting, but you'd enjoy a healthier imagination.'

'I didn't imagine it, sir!'

'Take it from me,' the Brigadier growled. 'A solid object can't just dematerialise.'

'The TARDIS won't dematerialise!' The Doctor wriggled inside the control console of his time-machine and scanned the components for malfunction with as much care as Lethbridge-Stewart had tended his ancient Humber.

Above him, the column b.u.mped and struggled like a worn-out beam engine, to the accompaniment of another ear-splitting alarm.

The Doctor crawled out from the console and re-entered the co-ordinates. But still the TARDIS refused to leave the ship.

Unlike the Doctor and his companions, Turlough was delighted to be aboard the ghostly s.p.a.cecraft. As he stepped from the transmat capsule, he surveyed the control centre as if it were the promised land.

He ran to the operations panel. The controls were unfamiliar, but he would soon get the hang of them.

He winced with sudden pain and his hand went to his side - there was something hard and burning in his pocket. The cube was glowing angrily as he took it out.

'Turlough!' came the voice of the dark stranger. 'The controls of this vessel are of no interest to you?'

'But it's a ship! I can get home!'

The crystal flared and the voice of the man in black grew more intransigent. 'I did not bring you here so that you could return home. Your concern is with the Doctor.'

But Turlough would not be held back now. In his impatience he felt strong enough to destroy the importunate old man. He raised his hand to dash the crystal against the hard floor.

He screamed. A terrible force issued from the cube, which seered his arm and tormented every nerve in his body. He writhed and twisted but could not dislodge the cube from his grasp. From the now-blinding radiance, the stranger burst, like the genie from the lamp.

'You will obey me in all things, Turlough!'

'Let me go.' Turlough cowered like an animal.

'Remember the agreement between us.'

The boy shivered miserably. 'Yes,' he stammered.

'You will seek out the Doctor and destroy him.'

In a tremulous whisper Turlough reaffirmed his allegiance. 'I will seek out the Doctor and destroy him.'

The light faded. The stranger was gone. And Turlough knew that the man who called himself his Guardian was evil.

'Turlough again!' muttered the Headmaster as he stood over the empty bed. It seemed that Lethbridge-Stewart had been right about the boy all along. 'I'm sorry, Headmaster,'

said Matron. 'He was missing when I came in with Doctor Runciman. And there's no sign of Ibbotson either.'

'I must talk to the Brigadier.'

'I've already sent a boy round to his quarters. But he's disappeared too.'

'Turlough!' shouted the Brigadier. 'Turlough!' He stamped moodily round the obelisk. Confounded boys, dragging him up this wretched hill. He wouldn't be surprised if it was all some practical joke.

'But sir, there was this sphere...'

'Ibbotson!' the Brigadier roared. But he didn't want to bring on one of his turns, so he breathed deeply, as Doctor Runciman had told him to, and marched silently off to search the woods.

It was the sound of grinding machinery that led Turlough to the TARDIS.

He stared at the police box as it struggled to dematerialise. It looked like an Earth object, but appeared to have dimensionally transcendental properties that no one from that planet could ever have designed.

The noise stopped as the machine stabilised. Turlough backed quickly into a dark corner as the door opened.

Could the young man who rushed out, followed by two girls, be the Doctor?

'Might have known,' muttered the Doctor to himself, as he rushed, like the White Rabbit, down one of the ship's interminable corridors.

'Where are we going?' Tegan and Nyssa scurried after him, determined not to let the Doctor out of their sight.

Had they spared a look behind, they might have seen a thin, pale youth slip out of the shadows and into the TARDIS.

'The transmat beam has been operated. The signal is interfering with the TARDIS.' As soon as they entered the control centre the Doctor made straight for the main systems panel.

'Look!' shouted Nyssa, who had seen the silver sphere in the previously empty recess. 'The capsule has returned.'

Tegan looked nervously round the room and out to the gloomy corridor that led to the rest of the ship. 'If that thing is back there could be somebody on board.'

But no one was listening. Nyssa had joined the Doctor, who was pulling the systems panel apart. 'The transmat signal is supposed to cut out when the capsule completes its journey,' he explained.

'Can you switch it off?'

'I hope so.'

'I hope so too,' added Tegan, peering over Nyssa's shoulder. 'I don't fancy a non-stop mystery tour of the galaxy.'

'Ah!' said the Doctor, with the enthusiasm of a householder who has just discovered extensive dry rot.

'You've found the fault?'

'In a manner of speaking.' He stood up and smiled, rather sheepishly, at the two girls. 'It's on Earth.'

Tegan's face fell.

'Come on,' cried the Doctor. 'Back to the TARDIS.'

Once more they trooped along the ornate walkways of the red ship. Neither Tegan nor Nyssa could see the point of returning to the time-machine since it was trapped by the transmat beam, which could only be switched off by going to Earth... in the TARDIS! They dared not ask the answer to the riddle. Neither could bear to think of being trapped forever on the ship.

Turlough was enthralled by the TARDIS control room. As he had suspected, the machine could travel in the fourth dimension.

A desperate idea came into his head. If he could travel in the TARDIS with the Doctor he could voyage back to a time before the dreadful pact with the man in black. He could break free of his bond with the evil stranger, yet still be liberated from Earth.

His head ached suddenly and violently. Perhaps the stranger knew his every thought? The very concept of disobedience must be erased from his mind.

Turlough laboured hard to unthink what had been thought. So great was his concentration that he did not hear the Doctor return.

The Doctor stood in the door of the control room, looking at the pale, frightened intruder. 'Who are you?' he said.

3.

An Old Friend Tegan never knew why the Doctor had swallowed Turlough's unlikely story of how he came to be in the TARDIS. Could it have been intuition? A fatalistic acceptance of the mesh of coincidence that was forming around him? Or was it remorse at the loss of Adric this sympathy for the strange young man who had broken into his time-machine?

Tegan didn't trust Turlough an inch. As if anyone from Earth would just walk into a transmat capsule! Though Nyssa was quick to point out that that was exactly what she had done when she walked into the Doctor's police box on the Barnet By-pa.s.s.

Perhaps, after all, the Doctor was just obsessed with escaping from the confines of the ship. 'All set. Earth 1983.' He finished setting the co-ordinates and moved to the doors.

'Where are you going?' asked Nyssa.

'Earth via the transmat capsule.'

'Is it safe?'

'It worked one way.' The Doctor smiled at Turlough, then turned back to the girls. 'Once I've disconnected the beam, the TARDIS, with you three on board, should follow me through to Earth.'

'Can I come with you?'

Tegan looked at Nyssa. Why was the boy so keen to stick with the Doctor?

The Doctor turned to Turlough, thought for a moment, then, to the surprise of the two companions, agreed to take the young man with him.

'Good luck!' shouted Nyssa, as the Doctor and Turlough left the TARDIS to go back to the capsule in the control centre.

'See you on Earth,' replied the Doctor confidently.

It was as well for Brigadier Lethbridge-Stewart that he had moved to the woods on the east of the obelisk in his search for Turlough. Had he chosen the other side of the hill, it would have done his health no good at all to have seen a silver sphere materialise between two trees.

The door of the capsule unsealed and the Doctor looked out. It was indeed Earth, and he and the boy appeared to be in one piece which was a relief; as he observed to Turlough, a badly maintained transmat system could do very nasty things to organic structures.

The small box in the Doctor's hand began to whistle and squeak like an elderly wireless trying to find the Home Service. The Doctor looked pleased with himself; they must be very near the beam transmitter.

As they walked from the trees towards the obelisk, the squawking of the detector grew more and more excited, until the Doctor stopped at the base of the monument. He ran his hands over the stonework and from a cavity withdrew a small canister. This was the device which had stranded the TARDIS on the red ship.

Turlough handed the Doctor the tool-kit he had been carrying for him. The Doctor selected a small instrument and knelt over the transmitter.

But the canister had no apparent opening, its casing being constructed entirely without welding or seam.

'Brute force, I'm afraid,' said the Doctor, extracting a more robust implement from the kit.

So engrossed was he in his task that he did not hear Turlough's sudden whimper. The boy had developed the most appalling headache. His view of the Doctor misted and blurred. Beads of sweat formed on his forehead, yet he shivered with intense cold. Something was gnawing its way through his skull. The invading genius spoke within him.

'Now boy! Do it now!'

But still Turlough knew that his new-found friend, kneeling so vulnerably on the ground in front of him, was a good man.

'In the name of all that is Evil, the Black Guardian orders you to destroy him now!' The voice resonated inside his head and Turlough was one with the evil stranger.

The boy picked up a boulder and raised it over the unsuspecting Doctor.

The Doctor reeled back, choking from the acrid smoke.