They run through alien cities holding hands and gasp with laughter against the TARDIS door. It's been a long time since the Doctor's life was like this, and she knows it cannot last.

Regenerating in Beacon Hills was an unpleasant surprise. Regenerating as a teenage girl was another, but she had said she'd wanted to leave her past behind, and the utter distaste she suddenly feels for her previous incarnations is nothing new.

She tries on a new wardrobe and an American accent and prepares to set off.

And then she meets the new girl in town.

"I'll miss traveling," Allison confesses over popcorn dipped in chocolate. Better than fish fingers and custard, she thinks with disgust. "I've gotten used to seeing new places before I can get bored, even if it meant leaving friends behind every time."

"Come with me," she offers. "There's room. And you can come back anytime."

The first thing Allison says in the TARDIS is, "Wow, you weren't kidding."

"No 'It's bigger on the inside!' or 'How is this possible?'" the Doctor asks, disappointed. "Not even a gasp?"

"I've had bigger surprises," she shrugs, and she plays with the dials and buttons before the Doctor can warn her not to touch anything.

It's fine. She's jaded. She craves the unexpected.

Allison starts the Argent tradition she already knows while they're hundreds of years in French history, and the Doctor pouts her lips and says, "The last time I met a werewolf, it was different."

She doesn't worry for herself. She's not human. She's immune.

A thousand years and companions she doesn't want to count have taught her to worry for Allison.

Allison reminds her of all of them. She hopes she's wrong.

Allison is in love with adventure, but not the Doctor, or so she thinks. No one has been for a long time. She's not sure if she misses it.

Allison kisses her cheek and spins on the way to her room on the TARDIS, and she wonders if she should be wondering.

Allison lives a year, two, three, and when she visits home she must feel like a stranger. They stop on Earth in her time for Allison to compete for a prize in archery, even if she'd get better practice visiting the Amazons, and at Allison's urging, the Doctor wins one in mathematics.

"What's wrong with setting society forward a couple of decades?" she says innocently.

The press paints them as a couple, the mysterious duo who've turned up suddenly and stayed for months, making waves, only to disappear as quickly. They live quietly because they have time.

The Doctor lets Allison fall asleep in her bed, and she watches her breathe, and need to.

Her mother dies, and Allison is on an icy planet with castles made of snow.

Her mother dies, and Allison can't go back, can't change it, and wants to, no matter how many time she is told it is fixed.

When she can't undo it, Allison takes her revenge, and the Doctor listens to the screams. Howls.

She hears the echo of Allison's "No!" before the sound of the TARDIS drowns her out.

She goes to the funeral, and she knew it was foolish to hope that Allison wouldn't notice her in the back.

"My aunt Kate died before I went with you," she says quietly. "It's when I found out. About everything."

"You were wrong to do it," she answers. "You were wrong to become her. You can't play god, Allison. You can't."

"I know," she says, but she doesn't. She can't.

She lets her come along anyway, because they're both lonely.

Allison leaves.

She tells her, "You're not like anyone else. You're the greatest person I've ever known. But I can't do this forever. You can. You literally have forever."

Allison reminds her of all of them.