Five years earlier, Chris stared through the railings at the playground's edge, looking out at a city that would no longer fall to the checkout machines. She'd still been so overwhelmed by everything that had happened in her day – the strange behaviour of the adults, with the nurse's the strangest of all – that she didn't even notice the four bullies approaching, pointing and giggling as she stared through the fence alone.

"Hey, Christina!" said the one who was tall and had braces. "Don't you have any friends anymore?"

Chris turned round vaguely, looking at the bullies like they were across a giant road. She'd somehow been terrified by them, even when she was being stalked by skulls. But something about that seemed faintly silly now.

"No," she said, "they all died. They burned to death, with their families. So I don't really have any friends anymore."

Two of the bullies looked unsettled then, like someone had been given the wrong script. But the one with the curious hair just laughed, in a way that meant she could bear her teeth as widely as possible.

"Did that make you sad, Christina? Did that make you cry to the woman you see 'cause you're crazy?"

"My Doctor said I wasn't crazy. She said the things I saw were real. And that everyone knew they were real, but they were all pretending, because they were too scared and afraid to say that they knew they were. But she said that it was better, to say when you were afraid. And if she'd have met you, she'd've said you should also be terrified."

The small bully laughed. "Terrified of what? Of you coming back and boring us with all the mad things that you say?"

Chris frowned. "Of this, I think. That someone could come and bully someone else, and there'd be nothing that anyone could do. Because they liked feeling strong, and knowing no one'll stop them. But we're children," she said, "and we're human," she added, which would never have occurred to her before. "Why wouldn't you be afraid? If something was coming to bully you, that wasn't a human child. That was stronger, and that was worse. And that no one'd keep you safe from it anymore."

The animals in the grass behind the fence were getting agitated, and the insects were buzzing in a slightly unusual way. The meteorologist that wasn't there would have been startled at what was happening with the wind, because it felt like something else that wasn't there might just be about to appear.

"We would have all burned, that night," said Chris to four children who were now feeling bullied themselves, "if it hadn't been for her. Everyone you love, and everything," she said. "All of it gone, 'cause nobody bothered to stop it. You're bullying me because it makes you feel powerful," she said with words that didn't feel like hers, "but you don't know what power really is. What it would do, if she were to abandon us. What'd happen to you, if there wasn't any good in the world."

"You're mad!" said the sparkly bully desperately, "and your shrink's mad too, and it shouldn't be allowed!" She was shouting above a noise that she was trying very hard not to hear, a high and throbbing sound that felt like it was coming from under the world. "You've got someone who's being paid to sit and tell you you're right to be crazy, and you're talking about her like you think that she's some sort of hero!"

"No. She was stupid, and arrogant, and thought she knew more than she did. And she hurt me, more than I think she knows. But I never said she was a hero," she shouted over a howI of noise, "I said she was my Doctor."

The wrongness in the air grew sharper until it was very wrong indeed, and a sound that was impossible to ignore was screaming against the wind. Even so, the bullies stared hard at the box taking shape before them, stared as if not believing hard enough would make that impossible thing go away. But the outline of the box grew ever more thick the more they stared and stared, until a battered blue bulk stood in the centre of the playground, abnormal in its strangely normal way.

The children stared up in horror, looking for sense of any kind.

"It's the police!" said one, pointing at the sign on the TARDIS door. "Run, you idiots! It's the police!"

The four of them turned and ran, having done just enough to convince themselves that the world was still all sane and normal. After a while the door of the box opened with a creak, and a small woman who was nothing like the police came out of it into the world.

The Doctor looked down at Chris, and gave her an awkward wave.

"It's you," said Chris, sulkily.

"It is. People must ask you a lot if you've had a nice time at school, right? But you probably haven't done that, today."

"Not really. Have you had a nice time in your box?"

"Oh, busy, busy. Saved the human race. I know it doesn't always seem like that's a good idea," she said, squinting at the receding bullies, "but it is. Trust me."

"I met you, too," she continued fter a while. "From the future. And she was very different, not just because she was a teenager. And I asked her how she felt when she was you, on this day that we're in right now. Because I made you think that I would know, and then I showed you that I really didn't. And that's not something you should do, to anyone who's given you your trust."

Chris looked at her feet. "I'm very angry with you," she said.

"Yes. And I'd expect you to be. I've spent so much time understanding so much; I didn't see… the things I didn't see. But I should have seen you. Because everyone deserves that, don't they? And you're part of everyone. So you do, too."

Chris kept looking at her feet. She shuffled one of her shoes around, grinding tiny stones against the ground.

"When you were older," said the Doctor eventually. "You said that… you did want to see it. All of time, and space. To be excited by it, and to cherish it. To be happy. And that if you thought that it was really, really something… that there was a chance one day you could forgive me."

"I might want that," said Chris. "But I have to stay with my mother. She gets so sad, and lonely. I need to be there, to make sure that she's safe."

"You won't need to worry about that," said a voice from inside the TARDIS as her mum emerged with a smile. "Thought we could both use a holiday, eh? After everything we've seen."

"Mum!" said Chris, horrified. "You can't come with the Doctor! She's my friend, not yours!"

"Want your own space, do you?" said her mum. "I understand. Not much fun to have your parents crash in on whatever you think is cool!"

"The Doctor's not cool," said Chris. "She's just my friend."

"I said to your Mum it wasn't safe," said the Doctor, ignoring them, "to go out there with me as a child. That I couldn't promise I'd keep you safe; that I might not be able to. I'm not an adult, at the end of the day. And I'm definitely not a responsible one. But your mum said—"

"—I said that it wasn't safe here. That we'd both almost died in the last few days, and all of the planet with us! I've seen what the future could be like, and what everything's like now, and I thought if the Doctor and me were both with you, maybe there'd be a chance you'd be safe—"

"I don't normally do this," said the Doctor. "Families. But you've both seen so much, and I've managed to let you both down. It's time I was the sort to make exceptions."

Chris looked at her mother, and at her eyes. She was smiling, but you could see that it wasn't quite perfect; that behind it there was something she'd never admit was desperation.

"I'll come," she said, old enough to know she was doing it for her mother, and young enough not to realise the lie. "But don't hurt me again."

"It won't be that sort of adventure," said her mother, "lots of fun in the sun; no more evil monsters trying to kill us. We can go to the pyramids!"

"We can do that anyway," said Chris, as she walked over the boundary that cut the TARDIS insides from the world.

"Yes, but now we can do it in a time machine! With a forest! And proper beds!"

"I never got to see the beds," said Chris with a smile.

"And you will now," said the Doctor. "Now, you might think a flowerbed is just something you find in a garden, but a few lives ago I thought there might be something rather more literal I could try—"

The Doctor closed the TARDIS door behind her with a thump, and the world pretended to be normal once again.

A Cyberman would have felt nothing as the cracked light of the police box flickered on, and its chipped and splattered frame began to fade away. It would have felt no thrill at the knowledge there were so many worlds beyond its own, and no excitement at knowing how people would soon catch a sight of a tiny few. And it would have been calm, and untroubled, and it would have lived forever. But that was not the choice that Lorna and Chris had made.

Human experience is very, very broad, and it now stood to grow a little broader…

...the TARDIS faded to nothing in the early evening sky.