It was clear from Chris’s face that she was prepared to be disappointed as she opened her present from the Doctor, but she somehow managed to look even more glum as she pulled it from out of the wrapping.

“It’s a tin,” she said.

“It’s what’s in the tin that counts! That’s a Paradox in a Can; open it and it sets off a bomb that blows the whole thing up ten seconds before you tried. Very popular in Time Lord nurseries, before they all got banned.”

“A bomb?!” gasped Lorna. “You’ve given my child something that’ll explode?”.

“That is a philosophically difficult question to answer,” said the Doctor, “but the bomb is just for starters. We’re going on a day trip as well, to somewhere nothing will explode at all!”

“I wanted a party,” said Chris.

“Well, you made a lot of inappropriate sound and light. That’s basically a party. But where we’re going is somewhere that you’ll love.”

She grinned.

“Have you been…” she smiled, “...to Edinburgh?”

“Once,” said Lorna. “We were going to go to the castle, but it was too expensive. Then we were going to go the zoo, but it was even more expensive than that.”

“Well, the place we’re going has a castle that’s also a zoo! ‘Cos we’re going to the exciting city–”

She whipped herself round her console dramatically.

–“of Edinburgh Four.”

Lorna and Chris stared at her for a while.

“...Edinburgh Four?” said Lorna in the end.

“It’s in space! You see, way in the future when Scotland went up in a rocket”–

“No, I mean. What happened to the first one?”

The Doctor looked tense. “I, ah. I probably shouldn’t answer that.”

“The second one?”

“I definitely shouldn’t answer that. But I can tell you what happened to the third one!”

“What?”

“Nobody knows,” said the Doctor cheerfully. “But you see, Edinburgh Four is special. It’s built like the first one, all cobbles and spires, but in a sort of wedding cake that all just floats, that can be seen for miles around! When they put together the list of the Earth’s best cities five billion years from now, it comfortably makes it into the top three thousand. And that might not sound impressive, but five billion years is really a very long time”–

“Does it have a hill?” said Chris.

“What? It’s in space! You can’t just go putting a hill up there!”

“The Edinburgh we went to had a hill,” said Chris. “We went up it, because it cost a lot less than the zoo. It had just been on fire, but it was fine when we were there.”

“Well, this Edinburgh has aliens, and holographic tartan, and this game called ‘Mega Pinball’ which I’m not sure is especially safe”–

“I want to go up a hill,” said Chris.

“This is a time machine,” said the Doctor through clenched teeth. “You don’t need a time machine to go up a hill!”

“There must be some good hills, though,” said Lorna. “In time and space.”

“Edinburgh Four, everyone,” said the Doctor as the TARDIS wheezed and groaned. “Try not to get too excited by it.”

The white door at the edge of the wood swung open, and they stepped out onto the cobbles of a city suspended in space.