The possibility that memories that aren't part of our conscious thoughts might still affect our behaviour has long intrigued doctors and scientists. In the late nineteenth century, Austrian neurologist Sigmund Freud proposed that shameful or traumatic memories that people hid from themselves might be the reason that they experienced mental health problems and unexplained physical symptoms. He argued that this process of repression made people unwell, and bringing such things to light through a new kind of treatment psychoa.n.a.lysis could be helpful. Psychologists and neuroscientists have moved on from Freud's theories, but they are still influential in clinical practice and in the way writers think about personality and memory, as we shall see shortly.

A number of conditions can affect our memories. The general term for memory loss is 'amnesia'. Memory loss is not uncommon clinically. It might be a short-lived process (for example, following mild brain injury), permanent as in the case of Henry Molaison, or progressive, in which case patients are said to have dementia. In many cases we can find anatomical damage that explains the memory loss, but sometimes there seems to be a psychological explanation.

Some treatments for these conditions use the a.s.sociative nature of memory. For example, Henry Molaison was able to modify some of the long-term memories he retained from before his operation to include new information less creating new memories than adapting old ones.

Something similar might be happening in The End of Time (20092010). In Journey's End (2008), the Doctor saves the life of his companion Donna by removing her memories of him. But those memories are not destroyed it seems he only removes her ability to access them. In The End of Time, when she sees people around her turning into copies of the Master, it reminds her of things she's seen before. Her brain processes the new experience by comparing it to previous memories and that need to a.s.sociate new memories to old ones is strong enough to bring everything back.

Importantly, our memories what we learn and experience make us who we are. In Chapter 7, we saw what the Doctor looks for in potential companions: they need to have the right att.i.tude. As they travel in the TARDIS, he provides them with skills and experience, and makes them better people.

Heartbreakingly, Wilfrid Mott says just that to the Doctor in Journey's End, when Donna has lost all her memories.

'I had to wipe her mind completely. Every trace of me or the TARDIS, anything we did together, anywhere we went had to go.'

'All those wonderful things she did ...

But she was better with you.'

The Tenth Doctor and Wilfrid Mott, Journey's End (2008)

In fact, the suggestion is that memories can be used to change how we behave in the future.

Recent research has revealed some interesting quirks about implicit memory. So-called 'priming' involves tinkering with implicit memory in ways that will subsequently affect behaviour. For example, in the early 1970s, American psychologists David E. Meyer and Roger W. Schvaneveldt conducted an experiment where people were shown a series of groups of letters some of the groups with letters in a random order, and others where the letters spelled out an everyday word. The speed at which people recognised the everyday words was recorded.

It was found that people were faster at recognising the word 'nurse' if they had seen the word 'doctor' earlier in the series. This suggested that the word 'doctor' prepared or primed them to recognise a.s.sociated words.

Although the science of priming is still contested, the idea seems to have influenced Doctor Who. At the end of Listen, Clara meets and rea.s.sures a scared child who will apparently grow up to be the Doctor. She holds his ankle and talks to him soothingly, creating a memory and entirely changing his life to come.

'Fear is like a companion. A constant companion, always there. But that's OK, because fear can bring us together. Fear can bring you home. I'm going to leave you something, just so you'll always remember, fear makes companions of us all.'

Clara Oswald, Listen (2014)

The First Doctor will later (for him) quote the words 'Fear makes companions of all of us' to his new companions in the very first Doctor Who story, An Unearthly Child (1963) and goes on to say that fear lives with another sensation: hope. Hope despite fear seems key to the Doctor's character and it might come from one brief childhood experience.

Memory is not just something we have but something we do: encoding memories, storing them, recalling them and acting on them. Remove our memories and we become different people. In Into the Dalek (2014), we learn that Daleks shut off certain memories to extinguish any glimmer of kindness or compa.s.sion, making themselves more ruthless. The Doctor gives the Dalek back one of its memories of a star being born and the Dalek feels a strong emotion, speaking of the star's 'beauty'.

At the end of the story, the Dalek rejoins the other Daleks but with 'unfinished work' to do. Because the Doctor changing its memory has changed this Dalek's whole personality. It has switched sides, and we are left to wonder what damage it will wreak among the rest of the Dalek race. The Doctor has used memory and emotion as weapons against his worst enemies.

But once, he and his people fought the Daleks using time...

The universe convulsed. For a moment, time itself held its breath. Then it let it out again in a chaotic amalgam of past, present and future. Might-have-been and could-have-done crashed into what-if and never-was.

Caught at the edge of the temporal wave, the TARDIS rode out the storm. The first the Doctor knew of it was when the ground dropped from under his feet and his head cracked into the console. When he woke a few moments later, he was lying on the floor. Until recently he'd had a ma.s.s of dark curly hair that might perhaps have cushioned the blow. But now his hair was cut short like the soldier he resolutely refused to be in this most catastrophic of wars. The lights fizzed and flickered like an electric storm. The Cloister Bell sounded cracked and melancholy. He dragged himself back to his feet and surveyed the panels in front of him. Every warning light was blinking at him. Even several he was sure he'd never seen before.

High above his head, the roof of the TARDIS had faded into a view of the contorted star systems as they folded in on themselves. Stars went nova; black holes collapsed; planets collided and spun away into oblivion. Several whole galaxies vanished from history never having existed. The view and the readings from the instruments that were still working told him everything he needed to know.

'The Time Destructor,' he murmured, the words lost in the alarms. So the Daleks had finally got it working, finally deployed it in this seemingly endless war against his own people, the Time Lords. Even here at the edge of the blast sphere it was impossible to predict the effects of such a device. But one very obvious effect was that the TARDIS was out of control. Crashing.

Desperately, the Doctor scanned the immediate area. He was in real s.p.a.ce at least. It would take a while for the Vortex to settle down again, Until it did, he was safer here. Wherever 'here' was. The instruments tried to match the alignment of stars to known constellations. Finally it settled on a probable location. But there were gaps. The Nestene Homeworld was gone, wiped away in the blink of an eye. Temporal shockwaves rippled out through systems from Grantaginus to Mellandrova, from the Farflung Rift to the Wolf's Heart Nebula.

Finally, he saw it. A tiny planet on the edge of the nearest system. It was impossible to know if it had been affected, but it seemed stable. For the moment. It would only take the merest hint of the ripple to turn the planet's sun into a supernova or a black hole, or an empty s.p.a.ce where no star had yet formed. But he needed to wait out the worst of it, and give the TARDIS time to recover.

Only as the materialisation circuits cut in, wheezing and complaining, did the Doctor realise that he needed something else. 'K7,' he read on the fault locator screen. The fluid links had ruptured. Wherever he was landing, he hoped he could find some mercury to replenish them. Another screen gave the name of the planet: Rontan 9. For the first time since the floor disappeared from beneath him, the Doctor smiled. It seemed he'd landed on his feet figuratively if not literally. He patted the console and reached for the door control.

It had taken Professor Targus Kornick years to develop the technology behind the Nihilism Chamber. It had taken him almost as long to book s.p.a.ce at the Rontan 9 facility and persuade the university to give him the time off. But now, at last, his dreams were coming to fruition.

'Is the field holding?' he asked.

Lizbet Harkening, Kornick's deputy nodded. 'One hundred per cent.'

'We have total isolation,' Dalla Fronstat, one of the students, confirmed.

The other student, Archan Noon, was drumming his fingers in an annoying rhythm on the workbench.

Kornick adjusted his spectacles, an anachronistic affectation which he believed imbued him with authority. He cleared his throat. 'Then for the next three months,' he announced proudly, 'we are completely shielded from any external interference or influence. Our experiments will be the first the very first ever, anywhere to be conducted in an environment devoid of cosmic ray activity, electromagnetic radiation, stellar neutrinos, gravitational waves, or any other transmissions or incursions. Nothing, absolutely nothing, can get inside this Nihilism Chamber.'

If he said anything else, his words were drowned out by the ear-rending sc.r.a.pe of noise that announced the arrival of a large, blue box in front of him.

If the Doctor was disappointed by the reception he received, he contrived not to show it. He grinned and waved, he b.u.t.toned and then unb.u.t.toned his velvet jacket and ran a hand through his hair as if it was still as long and curly as it had once been.

'How the h.e.l.l did you get in here?' an elderly man with grey hair and spectacles demanded.

'I came out of that box,' the Doctor told him. 'You saw me, just now, remember? Now then, I wonder if you have any mercury I could beg off you?'

'Mercury?' The man's voice rose an octave.

'This is Rontan 9, right?' the Doctor said. 'The facility where scientists from all over the system can rent laboratory s.p.a.ce to conduct their experiments away from the prying eyes and ears and wallets of large corporations, the press, and disappointed husbands and wives waiting at lonely dinner tables, yes?'

'Yes,' the middle-aged woman standing beside the man agreed. 'But we don't have any mercury, Not in here.'

'Out there, then?' the Doctor suggested, gesturing towards a heavy airlock-style door at the back of the room.

'We're not allowed out there.' It was one of the youngsters who spoke. Well, the Doctor thought, she was probably in her early twenties. A student, he guessed as was the young man sitting on a metal stool beside her.

'Not for months,' the young man added. 'Because this whole laboratory is a Nihilism Chamber so nothing at all can get in or out.' He glanced at the elderly scientist and stifled a smile. 'Except mercury hunters in big blue boxes, apparently.'

'Ah.' Well, that explained why he wasn't being welcomed effusively. 'Sorry about that. But the good news is that you're probably shielded from the temporal effects in here, even if you're not immune to the incursion of a relative continuum stabiliser in materialisation mode.'

The older scientist's anger seemed to have become a grudging acceptance. His eyes narrowed behind the lenses of his spectacles. 'What temporal effects?'

'Oh sorry, getting ahead of myself. The Daleks have deployed a Time Destructor.' He regarded their blank expressions. 'Great Time War, no? Pa.s.sed you by? OK then, maybe I've dropped back a few centuries in the blast wave, no problem. But I still need mercury.'

'And we still don't have any,' the woman told him. 'Not in here.'

'We'll have to start again,' the scientist said with a frustrated sigh. 'We can't conduct non-interference experiments with dirty great boxes and grinning maniacs just appearing out of thin air all the time. Who are you, anyway?' he demanded.

'I'm the Doctor,' the Doctor told him. 'And I don't want to alarm you, but we really should check that the world outside this laboratory still exists.'

The scientists introduced themselves to the Doctor while they shut down the various isolation field generators and withdrew the energy shutters. Finally, Professor Kornick unlocked and opened the main doors.

The world outside the chamber did still exist. But it had changed.

They picked their way through the debris, the two older scientists confused and the younger students pale with shock. The door from the chamber opened into a corridor, which led to a reception area. The receptionist was sprawled across her desk, uniform and flesh ripped open. The chairs were overturned, deep gouges scratched down the walls.

There were more bodies in the adjoining areas. The lights that were still working flickered erratically.

'What could have done this?' Kornick asked.

'Some savage animal?' Lizbet Harkening suggested, her voice hoa.r.s.e with nerves. 'Loose within the facility?'

'Let's hope so,' the Doctor murmured. The alternatives were even more frightening.

'What's that?' Dalla was staring at an open doorway. 'Can you hear that?'

'I can,' Archan said. He put an arm round Dalla, pulling her gently back.

The Doctor could hear it too. A low rumbling sound like an approaching storm. Or an animal. He took a step towards the doorway, watching as the shadows beyond coalesced into a shape huge, hairy, savage.

The rumble became a roar as the creature appeared in the doorway, like an upright wolf. The face was a snarling ma.s.s of hair and teeth punctuated by deep-set eyes and a dark snout. Claws ripped through the air as the beast bounded towards them.

Kornick backed away, white-faced. Lizbet grabbed him, and pulled him clear as the creature leaped. The students, Dalla and Archan ducked behind an overturned table. Only the Doctor stood his ground, staring in horrified recognition at the matted fur bursting through the remains of a white lab coat. The deep red eyes fixed on him, and the beast charged forward.

He grabbed a chair, its metal legs twisted and rusty. He swung it in front of him, like a lion tamer. The creature ignored it, kept coming leaped towards the Doctor, who tried to parry it swinging like a cricketer. Chair and fur collided and both hurtled off at a tangent, crashed into the wall, and slid down in a crumpled heap. The beast's claws scrabbled for a moment on the floor, then were still.

'I think it's out cold,' Kornick said, pushing his gla.s.ses up his nose so as to see better.

The Doctor caught his arm. 'I know you're curious, but it might wake up again.'

'I just want to know what it is. How could it have got in here? There are no animals like that on Rontan.'

'There are now,' the Doctor told him. 'Come on.'

A gla.s.sed-in walkway led to the next building. The gla.s.s was warped and discoloured. The Doctor paused to examine the metal struts that held the structure together. It was corroded, dull with age.

'How old is this facility?' he asked.

'They had their tenth anniversary last year,' Lizbet told him. 'But this looks... ancient.'

Before the Doctor could reply, Archan called back from further along the walkway. 'Come and look at this. It's...' His voice tailed off, as he failed to find the words to describe it.

The young student was peering through the discoloured gla.s.s. The landscape beyond appeared and vanished with the flickering light above them their own reflected faces staring back at them intermittently.

'It ought to be daylight out there,' Dalla said. 'We should be able to see the accommodation block.'

'So where is it?' Kornick said.

'I think that is the accommodation block,' the Doctor told them. They stared out at the flickering view of the crumpled steel supports and the rubble strewn across the landscape.

The walkway gradually became darker as more lights failed. The Doctor's sonic screwdriver cast a pale blue glow ahead of them, enough to see that the structure ended as if it had been bitten off. Their feet crunched on old, brittle gla.s.s as they walked slowly onwards.

'Time distortion,' the Doctor told them.