The Doctor folded his arms and turned away. Such was his petulance that Peri expected him to mince out of the console room.

Instead, he said, 'You never cease to amaze me. You are asking me to revive a man who had every intention of terminating my life.'

'That's right.'

Peri leant forward, tugged at a plastic tag attached to the tunic of Hugo's jacket until it was free and held it up for the Doctor to read: Lieutenant Hugo Long, Intergalactic Task Force, 'A' Squadron.

'Oh ...' said the Doctor at last. 'A policeman.'

'That's right. Now get to work and make him well again!'

Reluctantly the Doctor bent down and continued his ministration.

The Time Lord was puzzled. He was aware that he was having lapses of memory, but couldn't understand why Peri was being so aggressive. Come to that, he couldn't remember why they had come to t.i.tan Three. Perhaps the two things were linked.

In fact, the more the Doctor thought about the general situation, the more confused he became. Why had Hugo accused him of destroying his squadron? And who were the children he seemed so concerned about? Come to that, what was Hugo doing so far from his home planet? He couldn't imagine that t.i.tan Three was part of his normal beat.

The more the Doctor wondered, the more he realised how confused and muddled his mind was. He would have to do something about it.

But what?

6.

AN UNSAFE SAFE HOUSE.

t.i.tan Three has always been accused of being the bleakest, most miserable planet in the universe. Scenically, it is no bleaker than any other small planet devoid of vegetation. The real problem with t.i.tan is that its thin atmosphere contains a very rare gas nicknamed t.i.tan Melancholia. It isn't at all poisonous, but prolonged inhalation can cause depression in humanoid life forms.

Azmael had discovered t.i.tan Three while searching for somewhere to live after his self-imposed exile from Gallifrey. At that time he very much wanted to be alone and t.i.tan seemed to offer him precisely that.

He had been surprised when he had discovered buildings on the planet. And even more surprised when he had learnt they had been built by people from a nearby solar system that no longer existed.

It wasn't until he discovered a still functioning computer that he learnt of their sad fate.

Originally the buildings had been erected to house a research unit and monitoring base for the solar system, Maston Viva. Before building the centre, all the usual checks and tests had been made, including a close examination of the atmosphere. Although a gas unknown to the Mastons (t.i.tan Melancholia) had been detected, exhaustive research seemed to indicate it was inert and safe to breathe. So the centre was built.

It wasn't until some time later that it was noticed that people who spent more than six months on the planet became strangely depressed. At first this was dismissed as nothing more than an over-reaction to t.i.tan's bleak environment, so the tour of duty was shortened to three months.

This did little to help.

Scientist, technician and labourer alike started to abandon their work in favour of writing long, introverted, painfully self-critical novels and essays. When summoned home, they refused to go, preferring to stay on t.i.tan to complete their self-imposed tasks.

Such was the all pervading gloom of the place that Mein Kampf and the works of Strindberg were read as light comic relief.

It was during one of these intense periods of introspection that an enormous burst of radiation wiped out the population of Maston Viva. The scientists, whose function it was to warn of such impending disasters, were mortified. On checking their computers, they found that the radiation cloud had been visible for days, and if they had been more attentive to their duties, the danger could have been neutralised.

Suddenly, the pain of life had overtaken the agony of art. There was little left for the scientists to do. After each of them had completed a long, soul-searching autobiography, they committed ma.s.s suicide.

They were the first and last victims of t.i.tan Melancholia. Shortly afterwards, it was discovered that a daily gla.s.s of Voxnic acted as the perfect antidote to the side effect of the gas. But such were the terrible events that had taken place on the planet that n.o.body wanted to live there.

Originally Azmael had earmarked the planet as a bolt-hole in case the High Council of Gallifrey had changed its mind and again sent a squad of Seedle Warriors to kill him.

But that was a long time ago.

Nowadays, Seedle Warriors seemed relatively harmless compared with the paranoid ambition displayed by Mestor.

Yet here he was again, this time watching two immature boys struggle with chalk and blackboard to complete equations that had been set for them.

The twins weren't happy, being unused to such primitive implements. Their fingers were sore from holding the chalk and their arms ached from the effort of scratching their calculations on the squeaky blackboard. Although they had complained bitterly, Azmael had shown little sympathy. 'You've brought this on yourself. If you hadn't rigged that silly distress beacon aboard my ship, I would have let you use the computer... Now I can't trust you.'

The twins worked on, but they were running out of patience. The drug which controlled their minds was beginning to weaken, and their stubbornness was returning.

'There's no point to what we're doing,' complained Remus.

That's right,' echoed Romulus. 'Why don't you tell us what this is about? The equations you've set us could be done by an idiot. You don't need us for this sort of work.'

Azmael nodded. Romulus was absolutely right. What they had been given to do was simply to test their cooperation and the accuracy of their work. Mestor had insisted.

'To be honest. I do not know what is intended for you. You must understand that I am also a prisoner. I must do as I am told.'

The twins weren't certain whether to believe him. 'Then tell us who your master is,' they said as one voice.

Cautiously, Azmael looked over his shoulder as though expecting to find Mestor listening. 'His name would mean nothing,' he said quietly. 'But understand that he is a creature of infinite ambition.'

Azmael glanced over his shoulder once more. 'He will use anything and anyone to gain his ends.'

'Including us?' said Romulus.

Azmael nodded. 'He requires the gift of your genius.'

'He shan't have it,' said Remus, cutting in. 'We shall fight him if necessary.'

As the boy spoke, a swirl of red light formed into a hologram of the most repulsive creature the twins had ever seen.

It was Mestor.

'Fight me!' his rasping voice boomed. 'Beware, boy ... So far, I have been prepared to put up with your childish obduracy. But no longer! Fail to obey me and 1 shall have your minds removed from your bodies and use them as I wish... Do you understand?'

Terrified, the twins nodded. As they did, the image of Mestor faded.

'I did try to warn you,' said Azmael. 'Believe what Mestor says. He does not make idle threats.'

Lieutenant Hugo Lang lay prostrate on the floor of the TARDIS console room, his wounds dressed, a pillow under his head and a blanket covering his body. He looked cosy and snug, which is more than the Doctor did.

Something was agitating him.

Peri watched, as the restless Time Lord paced up and down like a caged tiger, and feared what he might do next.

'Something's very wrong.' The Doctor's voice had changed slightly, his diction had become more precise. Peri wondered who he thought he was this time. 'As a rule, most deduction is elementary, requiring little more than the application of logic. But to be honest,'he continued, indicating Hugo, 'the current situation has me baffled. Something is very amiss, my dear Peri. I sense evil at work.'

'The lieutenant isn't evil.'

'I'm not talking about him.'

'Then who?'

'The person behind the reason that brought him here.'

Peri was not only becoming confused, but concerned. She didn't want the Doctor to become involved in yet more trouble.

'Can't we just leave?' she said plaintively. 'Whatever may be going on here doesn't concern us.'

'It certainly does.' The Doctor paused in his pacing. 'My very being exists to solve crimes. I have spent a lifetime developing my powers of observation. Married to my unerring sense of logic, I have refined the routine of criminal investigation to that of a science!'

Suddenly Peri knew who the Doctor thought he was: Sherlock Holmes. How long, she wondered, would it be before he was racing across the planet looking for Professor Moriarty?

'You must understand my need to get to the bottom of this business.'

How could she? The Doctor wasn't Sherlock Holmes, neither were they in Victorian London.

'Even as a child, my gift was well-developed. With the use of pure logic and observation I deduced where babies came from.'

Peri yawned, hoping it would distract the Doctor from his fantasy.

But if he noticed he didn't respond, continuing as though every word was true.

'My mother had always insisted that the stork brought babies, but living in a large city I found that difficult to believe, as the arrival of infants was frequent but the sighting of storks was very rare. In fact, it wasn't until the age of ten that I saw my first stork - and that was in a zoo!'

'So once and for all I decided to solve the mystery. Word had it that a baby was due next door, so I set about watching our neighbour's house. Apart from someone called a mid-wife, no-one else entered or left that dwelling until I heard the cry of a new-born babe.'

'No stork either, huh?'

'Not even a sparrow. Now it had not gone unnoticed by me that the mid-wife had arrived carrying a large satchel. She had no sooner entered the house than I had heard an infant crying. I therefore deduced that the mid-wife had brought the baby in her bag!'

A bemused smile spread across Peri's face. 'Brilliant. A very clever deduction for an unenlightened child ...' She paused. The look on the Doctor's face told her that she had said the wrong thing.

'So I was wrong on that occasion!' he shouted. T couldn't help it if my idiot parents had refused to tell me the facts of life ...'

'But if you were mistaken once, you could be again.'

The Doctor had started to sulk. 'Rubbish! I have since perfected my method.'

Peri gave up. What could she say to a man, who in the s.p.a.ce of a few hours, had played Jack the Ripper, wanted to be a hermit, and was now basing his personality on a fictional detective? To argue seemed pointless.

She knew it could prove very dangerous, especially as something else had occurred to her which made her feel rather sick.

Up until now the Doctor had played at being rather silly, if not pathetic characters. True, he had tried to kill Peri and in his remorse had taken her to a remote, barren planet, which she had no way of leaving alone. Under more normal circ.u.mstances any single one of these acts would be considered outrageous. But this was far from any ordinary situation - the perpetrator was a Time Lord.

It was this fact, until now, that Peri had overlooked. What the Doctor had done was nothing compared to what he was ultimately capable of. All it required was one wrong word at a critical moment and he might convince himself he wanted to dominate the universe. Should that occur, then nothing would be safe. The Doctor's knowledge and ablity to manipulate time made it possible for him to change or destroy everything.

But how was she to prevent it?

As Peri thought about the problem, the Doctor started to pace up and down again. Perhaps the simplest way, she considered, would be to play along with him. If the Doctor wanted to be Sherlock Holmes, she would be his Watson. By acting out his fantasy he might begin to trust her. When the Doctor next tried to swap his personality, she might be able to control or direct him. With a little luck, she might also, gradually, lead him back to his 'real' self whatever or whoever that should turn out to be.

At least she could try.

'What are you going to do?' asked Peri, tentatively.

'Solve the riddle, of course!' said the Doctor, rummaging in his pocket.

For a moment she thought he was looking for his Meersham.

'Have you seen my jellybabies?'

Peri shook her head.

'It's just that I think much better when I'm chewing.' The Doctor smiled awkwardly. 'Never mind, I'll have to do without them.'

'You still haven't said how you plan to solve the riddle whatever it is.'

'First, we must consider the facts,' said the Doctor, crossing to Hugo. 'In spite of the fact that our young friend here has been shot down, he still has one other thought in his mind - the children.'

So far. Peri couldn't fault his logic.

'Now, let us a.s.sume he is here to find the children, and whoever has them, shoots him and the rest of his squadron down.'

Again, the Doctor's reasoning was sound.

Therefore, as Hugo crashed on this planet, one of two things could have happened. Number one: he was shot down while pursuing the abductor of the children. Number two: the abductor is based on this planet and destroyed the squadron as it approached.'

The Doctor crossed to the console and switched on the scanner-screen. 'I am inclined to believe that the villain is here on t.i.tan Three.'

'How come?' said Peri, trying hard not to sound too much like the traditional, dumb sidekick of a fictional detective. 'You said yourself there wasn't any life here.'

'There wasn't... But things change.' The Doctor pointed at Hugo.