"You know," said Kala as she surveyed what she'd carved into the asteroid, "when I asked if there was anything I could do, I didn't think you'd want something this demanding."
"People need to stay away from this place," said the Doctor. "Whatever they look like, and whoever they are."
"And this is going to help?"
"It's a Snapcode," said Chris. "Everyone's on Snapchat."
"You shouldn't be on that, Chris," said the Doctor.
Chris shrugged. "Everyone is."
"That's nice for everyone," said Kala. "But what is it?"
"It's a ghost," said the Doctor. "For something that's visible just for a moment, before it's gone away forever. And one day millions of years from now people'll look at it-–" she sighed "—and they'll say that it makes them feel old."
Kala was looking down at the Earth as it slowly turned. Up here, all the perils of the past and future seemed tiny. There was only the present ahead of her, and only one single world.
"You think it's all for you, don't you?" she said. "You don't ever notice you're doing it, but you do. You live in a world where everything's built for people; there are people on the billboard and smiling out of screens. You never even see how the people world with chairs and houses is a tiny part of that world"— she waved at the Earth —"of there." She smiled sadly. "It seems almost silly, now I'm here."
"You never felt it," said the Doctor. "You know it, but you didn't really know. Not in your body, not in the way that matters. And now you do, and so nothing is the same."
"You weren't going to tell me," said Kala.
"I didn't know for sure. What the oil really was; what the monster was. I had no idea about any of it."
"But you knew what happened to my people all along. And you didn't tell me."
"I wanted to," said Chris. "But the Doctor thought it'd be cruel."
"That seems like no one's place to say. It is cruel, of course. But I always thought if something like this was coming, I'd want to know. Maybe it wouldn't be better for me; maybe it'd change the way I saw all this—"
She waved one hand in the air, to indicate the Earth and the darkness of space. It was day over her planet now, and no trace of her civilisation was visible from far out in space.
"—but I'd want to know. It's treating me like a person, isn't it? And choosing not to tell me makes me think… that you don't trust people. Not to see the world as you think it is."
"Nobody wants to see that," said the Doctor.
"Perhaps," said Kala, "or perhaps you'd be surprised."
"Will you tell your son?" said Chris. "I would want to know, if I was him."
"I want to. But I wouldn't know what to say."
"You tell him the truth," said the Doctor. "That you'll do everything in your power to fight for him, because you love him even more than the world. But if that fight fails you'll love him all the more— the time you have together will matter more than anything could. People always say that children are the future, don't they? But it doesn't really matter if that's true. He's still your son; still worth cherishing. And you don't need to tell him he'll grow up to do good in the world, because he already has. And he already will," she said, "however the future might go."
Kala fell silent, looking down at the Earth in the void.
"You see?" she finally said. "You even knew the word. And you were stil the most scared of any of us, though you had the least to lose."
She bent down to Chris, trying to ignore how pink and strange she was.
"You'll take good care of it," she said. "Won't you?"
"I'm eleven. I don't get a say in how the whole planet's run! And we do it really badly, anyway."
"See, Doctor?" said Kala with a smile. "Your friend understands. Truth hurts, but telling it's still better. I hope she makes you see that in the end."
"Take care," said the Doctor softly.
"'Till the end of the world," said Kala.
She walked on over the horizon, to the place where her small ship stood.
"Funny," said the Doctor after Kala was far enough away. "A Snapchat. It's a thing you use up and throw away. And it'll be near the last thing left of her civilisation; the only way they'll know it was ever here." She looked down at her eleven year-old friend. "It makes me feel properly ancient."
"It makes me feel sad," said Chris.
"Yes," said the Doctor. "It does that to me, too."
She looked back down at the shapes of the ancient Earth.
"Best be getting back," she said. "Your mother'll be wondering where you are. I'd say we're a bit early for her yet, but I don't think we will be by the time we get you home."
"Hold my hand," said Chris, and the Doctor squeezed it tightly.
They walked the tiny distance to their Police Box, ready to jump to their tiny slice of time.