Author's Notes:
Slightly edited version of a prompt fill on Tumblr for human_nxture, originally posted to AO3.

She'd left the Doctor alone for half an hour, Peri thought, while she bought a new notebook. Half an hour by her well-maintained Earth watch, and a totally unexceptionable errand. Was it too much to ask that he avoid finding new and inventive kinds of trouble for just half an hour?

Apparently so. Because he was sitting on the steps of the forum hall, apparently cheerfully unaware of the crowd of diverse alien travellers and shoppers parting around him and nearly stepping on him, with what looked like a bag cradled in his arms.

She decided to walk up behind him and see if he noticed.

"Don't you worry, my love," she heard him cooing.

To the bag.

All right, then.

"Mummy will be back very soon, you'll see," he went on. He offered one hand to the bag, and a small purple tentacle reached up from it, curling around his finger. It wasn't a bag at all, she realized—it was a baby carrier, for... squid babies? Octopus, octopi, octopodes? Who could say.

She tapped the Doctor on the shoulder, and he jumped and turned his head, still cradling the baby squid (if it was a squid). Standing several steps above him, while he was sitting down, she felt she very definitely held the high ground for once, and she intended to use it. "Mind explaining?" she said.

"Ah!" said the Doctor. He looked slightly guilty, which was in some ways more worrying than if he had looked altogether guilty. "Well, you see, there was this lovely Askelcha woman..."

"That would be a squid person?" prompted Peri.

"They're not squid people, Peri, that's very offensive, they're—and anyway would you not interrupt me?"

"Sorry. There was this Asquelcher woman..."

"Askelcha," said the Doctor. "She had some legal business to do in the forum and she asked me to look after her larva while she does it. She said it would only take a few minutes."

Peri crossed her arms. "And she made big sad eyes at you and you said of course you would do it." On second thoughts, maybe it wasn't the best idea to make the Doctor aware that this always worked on him. She found it quite useful herself.

She sat down next to the Doctor, although still one step higher, and bent over to see the larva. It reached another tentacle out from its water-filled cradle, and she couldn't help meeting the tentacle with her finger. It was surprisingly soft, although it tickled slightly as it wound around her finger.

"Did she say the baby's name?" she asked.

"No," said the Doctor, "she seemed to be in quite a hurry."

Peri paused and remembered something she'd seen on the news while shopping. "Is there any chance," she asked, "that the woman you talked to was the notorious con artist Nasker Loosh? Tall, sort of purple and blue with tentacles and one big eye?"

"Sounds like her, but I wouldn't know. Why do you ask?" said the Doctor.

"Because she was sentenced to five years in prison about ten minutes ago in the forum," said Peri. "Looks like her 'legal business' didn't go so well."

The Doctor met her eyes with an expression of dismay.

"We might be babysitting for slightly longer than we anticipated," she said.

"What are we going to do?" said the Doctor.

"Well," said Peri dubiously, "I suppose we ought to tell the authorities."

The Doctor looked sulky and reluctant.

"I mean, we can't keep it for five years," she said. "You know we can't. Perhaps there's a foster system, or it's meant to be staying with its nearest relative or something."

He held the baby protectively close to himself. "I've never been to this planet before, I know very little about it. I'd like to be sure the system is any good, first. Surely she wouldn't have given her baby to a stranger if she had any better options."

"You never know," said Peri. "She's a con artist. It could be part of a scheme." She ruffled the Doctor's curly hair—one of the small pleasures of sitting on steps was being able to reach his head. "Look. There was a law section in that bookstore I went to, there's probably a book on family law. We can find out what the situation really is, and then we'll decide what to do. All right?"

"Good thinking," he said. He stood up, still carefully cradling the baby carrier.

"And you can cuddle it all you like while we do. Just—let's not make more trouble, OK?"

"You're probably right. Oh well," said the Doctor, happy again in his mercurial way. "At least the baby likes me. Don't you, my darling?" he asked the larva, and Peri sighed deeply. But she couldn't help smiling as well.