'Where did they go?' Where do dreams go? 'They were childhood stories,' she says, as if that should be enough explanation. 'Things I told myself to help me fall asleep.'

Softly, the Doctor says, 'I think we both know that they were something more than that. And still are.'

Tilly thinks of the voice the latest voice. We're coming. Coming through... She drinks her coffee in one quick gulp. He has made short work of the big biscuit, she notices. 'I grew up. There were exams, university...'

The Doctor nods sagely. 'Lipstick. Parties. Girls. And boys.'

Tilly can't help laughing. 'Something like that, yes.'

'So you became less open to the messages. You didn't have the bandwidth. Busy time, being a teenager. Brain expanding. Going supernova. Learning about cause and effect and life and death and getting the first tiniest inkling that you're not, in fact, going to live for ever.' He's getting excited now, and he wags his hands about, sending biscuit crumbs everywhere. 'Big issues. Big deal. Needs a bigger brain. But even that bigger brain it's so full, all the time, too full, not enough room for everything any more-'

Tilly holds up a hand to stop the flood. 'Whoa there, mister!'

'Doctor.'

'Doctor-mister. Hold on a minute. Did you say bandwidth? Like some kind of receiver?'

'Yes, yes, a receiver!'

'You make me sound like a telephone! I'm not a receiver!'

'No?'

'No! I was the kind of child that had an overactive imagination. Daydreamed a lot. Probably read too much-'

'There's science to prove that's not possible,' the Doctor says, earnestly. 'Real science, of the best and most authoritative kind. And if there isn't then I'll do some. So there will be.'

'There'll be what?'

'Science. And proof.'

This has all, Tilly thinks, got quite out of hand. It's time to stop dreaming and time to stop making up stories. She is a grown-up now with grown-up responsibilities. There is a pram, with a baby asleep in it, and a landlord who likes regular rent, and on top of everything the washing machine has been making a funny noise the past few days and she simply can't manage without the washing machine. 'These are not messages,' she says firmly.

'Yes, they are. They always were. And now they've come back. But they're not how they used to be.'

Tilly shudders.

'Yes, back with a vengeance. Look at it this way,' he says. 'Either you're receiving messages from somewhere, or you're hearing voices inside your head. I know which I'd rather.' He leans towards her. Suddenly, the clatter and chatter of the cafe seem to recede, leaving them in their own quiet bubble of s.p.a.ce and time. 'I can help. I'm the Doctor. But you need to tell me about it. Tell me what you can see.'

Tilly puts her hands to her forehead to block him out. She shakes her head. She doesn't want to go back to that place.

'Have some big biscuit,' says the Doctor. 'It helps.'

The Scariest Place in the Universe As a child, she walked along these streets a hundred thousand times. She knew them when they were beautiful broad avenues lined with great trees, full of laughter and song. She watched the trees grow and age, and saw new ones planted and flourish. She heard the music swell and fall away and rise again. She watched fashions change in clothes, in buildings, in ideas and come round again. She saw growth, and decay, and new growth.

But now it is all the h.e.l.l times, all at once. It is the Book of Revelation. It is Dante's Inferno and dying Charn. It is the Waste Land, with red rock and without relief. Where once there was sweet music, now there is only mourning but not the order and transcendence of a requiem. There is no logic, no order to pain. There is only the present moment, an endless moment of agony...

'They're hurting each other,' Tilly whispers. Tears are sliding down her face. 'They were so wise, so kind. And now all they do is hurt each other. Why are they doing it? Why don't they stop?'

The Doctor doesn't answer (perhaps, Tilly thinks, there is no answer to those questions), but on the table between them something is going beep. Quietly, as if it doesn't want to make a fuss, but insistently, as if it knows its business is important. Tilly reaches out a finger to touch it, like a shipwrecked sailor bobbing in the ocean, grabbing something to stay afloat. 'What is this?' she whispers.

'I'm trying to track the source of these messages. Find out who's sending them, and why...' Beep beep beep. 'Could be an invasion, you see.'

'An invasion?'

'Could be. World ending. Looking for someone else to move. What's the word for that? Lebkuchen?'

'Lebensraum,' says Tilly. (Her degree happened a while ago now but had covered that kind of thing thoroughly.) 'Lebkuchen's a sort of biscuit.' Her hand reaches for the pram. 'Do you really think this is an invasion?'

'I don't know. But have to do something about it, if that's the case. Make a plan. Or eat a biscuit. Probably go with the plan plan first. Eat the biscuit after. If I can just work out where they're coming from...'

Beep beep beep.

For a minute or two he becomes so focused on his little machine that he seems oblivious to everything around him: the muzak, the chatter, the scared woman sitting opposite him. He is in his own world and she has the faintest impression that he has forgotten about her, and that her troubles have turned into a puzzle to be solved, with quick thinking and clever widgets.

'I'm frightened, Doctor,' she says.

He looks up at her. He reaches out and, tentatively, pats the hood of the pram, and clears his throat. 'Well, yes, of course you are,' he says. 'You're carrying the whole history of a civilisation in your head, and that history is ending. Coming all in a rush. Burning into your brain-'

'That's where you've stopped helping.'

'Have I? Sorry.' The beeping gets suddenly louder. A man with a suitcase at the next table looks at them curiously. 'Hush,' the Doctor says to the widget, and turns it off. He peers at it. 'Yes,' he says. 'There they are.' He looks at Tilly. A kindly, almost tender, smile spreads across his funny mismatched face. 'Do you want to meet the people who've been talking to you?'

'Shouldn't they be talking to someone else? Someone proper? A world leader, or something?'

'A world leader?' He looks at her in bafflement. 'I want us to be friends.'

'Fair enough. Will it be dangerous?'

'I don't think so. But I can't see the future. Only travel there.'

Tilly stands up. 'How do we get there? Through the wardrobe?'

'Near enough,' says the Doctor, and offers her his hand.

Tilly takes the measure of the blue box. 'What about the pram?'

'It'll fit,' he says, and opens the door to eternity.

The Most Beautiful Place in the Universe Here they are. Here they really are. And this world no longer has the translucence of dream, or the rounded edges of memory, but all the roughness of a real world.

'It's all true,' says the Doctor. 'You didn't make it up.'

'But how?' says Tilly. She touches the walls. They are as solid as the pram. 'How could that happen?'

'They were clever, once upon a time, the people that lived here,' the Doctor tells her. 'They loved their beautiful world, and their wise ways, and they wanted people to know about them. So they transmitted images from their lives sent them out to anyone who might listen and understand.'

'And I heard them.'

'Yes, you heard them, Tilly. Not daydreams. Memories.'

On the walls are pictures of a world that was lost; a world that Tilly once knew as well her own. 'What happened?' Tilly asks. 'What made them cruel instead of kind?'

'Who knows? We can guess, I suppose. Conjecture. Perhaps their sun began to die. Perhaps the water or the soil became poisoned. Perhaps they simply became too proud and forgot what had made them great.' His eyes are dark and limitless. 'That happens sometimes.'

They walk along the corridor. Their footsteps echo. This is a stark place, sterile and lonely. Outside the world is ending. But here, inside the bunker, someone is still working.

A woman sits behind a desk. Her hair falls in rats' tails about her face. She is thin and tired, but she is carrying on.

Tilly peers through the gla.s.s that separates them. 'Who is she? What is she doing?'

'I think,' says the Doctor, 'that she's been trying to tell you something.'

Tilly watches for a while and then, very gently, taps on the gla.s.s that separates them.

The woman looks up.

Tilly raises her hand and presses it against the gla.s.s. Slowly, the woman raises her own hand. In greeting.

'I hear you,' Tilly says, and although the other woman cannot hear, she smiles. She understands. 'I have always heard you.'

Here we are now, in the park. Here is the pram and here is the little one, still asleep. Over there, a father is pushing a child on a swing a smooth arc that goes up and down and back and the child is laughing.

'I wish we could have saved them,' Tilly says.

'So do I,' says the Doctor, after a pause. 'But sometimes I'm too late.'

There's a time machine, Tilly thinks, but even so there would have to be constants. It is not possible always to live in the present, not even as a trick of the mind. There must be change and growth and ending. That is the way the universe works.

'I don't want to forget them,' she says.

'You won't forget them. How could you forget them?'

'I don't want them to be forgotten.'

'Ah,' the Doctor says. 'Now that's something different.' He looks at her, curiously. 'So what will you do?'

She could tell people, she thinks. Tell them what she has seen and what she knows. And they'd laugh and tell her she's talking rubbish, or else look away awkwardly and start talking about house prices instead. There is a lot to discuss when it comes to house prices.

'You could tell her,' says the Doctor, and taps the pram. 'I bet she'd like to hear.'

And she could and she will. She'll tell the little girl, and maybe together they will write it down, and that way somebody will remember those wise people who lost their way. Once upon a time some people lived, and sometimes they were happy. That is all we know, she thinks, and all we need to know.

'A man is the sum of his memories, you know a Time Lord even more so.'

The Fifth Doctor, The Five Doctors (1983)

Moondust is clingy. Because there's no atmosphere on the Moon, the dust isn't weathered and smoothed as it is on Earth. Instead, the dust about half of it fragments of silicon dioxide gla.s.s created by meteor impacts remain sharp, just right for sticking to s.p.a.cesuits.

The astronauts who walked on the Moon between 1969 and 1972 brushed down their s.p.a.cesuits before returning to their s.p.a.cecraft, but some of the dust stuck fast and was carried inside. As a result, when the astronauts removed their s.p.a.ce helmets inside their ship, they got a whiff of the Moon. It smelt, they said, like gunpowder. Apollo 17 astronaut Jack Schmitt even had an allergic reaction to it a bit like hay fever.

It's odd learning what the Moon smells like that detail brings it vividly to life. Smells can often bring on sudden, strong memories and even change our moods. We understand the reason for this: the olfactory bulb (at the front of the brain, responsible for our sense of smell) is connected to the brain's limbic system. This limbic system includes the amygdala (which processes emotion) and the hippocampus (which learns a.s.sociations between things).

The ability of certain smells to bring back rich recollection is just one of the quirks of memory that psychology and neuroscience are beginning to explain. The things we've learned so far are often surprising and counterintuitive. For a start, it might not be accurate to think about so-called memory as a single ent.i.ty at all. It actually describes a diverse set of processes that help us to navigate our day to day world and literally make sense of our lives.

We can, however, make some broad generalisations to help us to think about memory. Information from the outside world needs to be encoded; it needs to be stored somewhere; and it needs to be retrieved when you need it. Disrupting any of these activities will interfere with memory, both in the real world of physical and mental health, and in the world of Doctor Who.

Encoding is the first stage of any memory formation process. We're constantly receiving information from our five senses our sight, hearing, smell, taste and touch and our brains seem to store these perceptions in detail for about half a second. Through a process called 'attention', our brains select parts of this sensory information over others to create our short-term memory.

You're using your short-term memory as you read these words. To understand this very sentence, you need to keep in your head the words at the beginning of the sentence while reading the rest of it. Our short-term memory seems able to hold about seven items of information in our heads for up to about fifteen seconds, allowing us to process information and make sense of the world around us. So you'll soon forget the first words of this sentence unless your brain thinks they will be useful to you in future, and stores them in your long-term memory.

What we remember is therefore not a video recording of the world, but a selection decided by conscious and unconscious processes. In his 1932 book Remembering, British psychologist Frederic Bartlett conducted an experiment where people were asked to read a story from traditions of the native people of Canada, called the 'War of Ghosts'. When Bartlett asked people to retell the story they'd read, he found that they omitted or reshaped elements they were less familiar with, changing the story to better mirror their own culture.

In Remembrance of the Daleks (1988), the Seventh Doctor and Ace battle Daleks in London in 1963. Ace, who is from London twenty-five years in the future, wonders why she hasn't heard of this battle as part of the history she grew up with. The Doctor asks her about other alien invasions of London featured in the series and she's not heard of those either. 'Your species has the most amazing capacity for self-deception,' he concludes as if people have simply chosen not to remember.

The Day of the Doctor (2013) suggests that the Doctor's friends at UNIT are actively involved in helping in this process. We learn that they tell the public that the arrival of the TARDIS in Trafalgar Square is a stunt by TV magician Derren Brown a cover story that will work because it better fits people's experience than an alien in a time-travelling police box.

The hippocampus seems to sort new information by comparing it with memories already recorded. We know this from studying people who have suffered damage to the hippocampus and have trouble creating new long-term memories. For example, in 1953, an American man called Henry Molaison who suffered from epilepsy and seizures underwent brain surgery which removed much of his hippocampus in an attempt to improve his condition. Afterwards, Molaison could no longer create new long-term memories. Until he died in 2008, Molaison enjoyed doing crosswords but his general knowledge was limited to things he'd learnt before he'd had the operation in 1953.

We can use the a.s.sociative nature of memory to our advantage when it comes to recalling facts and figures. We can remember complicated information by a.s.sociating it with something much simpler, using a device called a 'mnemonic' (from the Greek word for memory). For example, we might struggle to remember the order of the colours in a spectrum, such as a rainbow red, orange, yellow, green, blue, indigo and violet. But it's much easier to recall that sequence using a simple mnemonic sentence using the same initial letters 'Richard of York gave battle in vain.'

But, as we've already mentioned, our brains don't only encode and store those memories that we consciously want to remember. Imagine you're a child taking part in a school sports day. You're out on the playing field which has been newly mown and the air has a tangy stink of cut gra.s.s. (We think plants such as gra.s.s release strongly smelling chemicals when they're attacked as a defence mechanism against insects and that they can't tell the difference between an insect and a lawnmower.) Your brain remembers this sports day. Because of the way the limbic system is connected, the hippocampus links the memory to the smell that the olfactory bulb detects the cut gra.s.s and to how the amygdala says you're feeling.

Years later, you're out for a walk on a nice, sunny day and catch a whiff of cut gra.s.s. In an instant, your brain connects that smell to the memory and emotion it has stored away. Perhaps you immediately recall standing on the playing field that particular sports day. Or perhaps it's just the emotion that hits you: it's a lovely day where you are now but you suddenly feel sad because all those years ago you didn't win your race. Sensations like this are called 'involuntary memory', named by the French novelist Marcel Proust in his famous book, In Search of Lost Time (first published between 1913 and 1927).

The people making Doctor Who sometimes use this quality of smell for dramatic effect. In The Doctor's Wife (2011), Amy must think of the smell of dust after the rain to open a locked door on the TARDIS. In The Krotons (19681969), the arrival of the TARDIS on an alien world was recorded in a quarry in Malvern one that looks little different from the many other quarries used as planets in the show. However, without expensive special effects, the writer of The Krotons Robert Holmes makes this planet seem more interesting by giving us two contradictory responses to it.