A/N: Started out as a riff on the Librarian-in-the-TARDIS-library theme that pops up now and again, and took on a slight mind of its own. Enjoy!
...
"Er, Doctor?"
The Time Lord thus addressed rolled over, his time sense kicking in as he lurched towards consciousness and informing him it was relatively two in the morning. "Cannit wait, Rose?" he mumbled blearily, Northern accent thickened in his half-asleep state.
Apparently not. "There's an ape on board, an' I don't remember it being here before," Rose pressed.
He couldn't be hearing her right. "'M I rubbin' off on you, Rose?" he mumbled. "Thought you didn't like me callin' humans that."
"I don't, but this is an actual ape," Rose said. "Big and orange and hairy and all. Brought its own banana. Goes 'Ook' when I talk to it. Is there a menagerie on board that it escaped from? 'Cos I thought nothing could get on board the TARDIS unless you let it on."
...
Rose had known the cascade of thuds from the TARDIS library needed to be investigated, because she'd heard them. The ship, she strongly suspected, shuffled any rooms that weren't constantly used around at whim, which meant Rose only heard commotions and disturbances, especially late at night, if the TARDIS had moved the noise's location closer to her on purpose.
The last time she'd been woken in the night, the Doctor had been having a vicious nightmare, something she'd discovered because his room had turned up next door to hers instead of a couple of corridors away. She'd gone and woken him out of it before slipping away, and in the morning his room had been back where it was supposed to be. He didn't seem to remember the incident, and Rose had chosen not to bring it up.
Thinking on this, she shoved her feet into slippers and crept down the hall, discovering the library door standing ajar with the light on inside. A quick look inside revealed nothing out of the ordinary, although there were several books stacked in the middle of the floor where none had been before.
Rose took a few steps inside and heard a soft shuffling along the carpet. A short, hairy figure came around a bookshelf, a book under each long arm, and blinked at her in the dim light.
"Ook," it said.
Rose decided it was time to involve the Doctor.
...
"I thought nothing could get on board the TARDIS unless you let it on."
The idea punctured through the Doctor's mental haze that Rose was frightened. Not surprising, he supposed; the TARDIS was supposed to be a safe place, and she'd just been startled by a strange creature in... "Where'd you say it was?" he said thickly, starting to sit up.
"In the library," Rose answered. "I heard something in there an'..."
The Doctor flopped back down, relieved. "Tha's all right then," he promised. "He does that."
"He?" Rose sounded like she wanted an explanation, but now that he knew there wasn't any immediate danger, he wasn't going to give her one till what passed for morning in the Time Vortex.
"Yeah," he said, turning back over. "He comes an' goes, he doesn't bother anybody, an' he's not dangerous unless you call him a monkey. He'll be gone in the morning; I'll explain then. Go back to bed, Rose."
He was asleep again before he could listen for Rose leaving.
...
She greeted him with breakfast when he reached the kitchen, her way of insisting on a full explanation. He accepted, sitting down, and began as he reached for a mug of coffee.
"He's called The Librarian, the one you found last night," the Doctor started. "Used to be a human, or something like one, but he doesn't talk about it. Wherever he's from, it's a long way from anywhere I've traveled to. I don' think he travels the way we'd understand it.
"First time I caught him at it, I made him explain, an' he said he travels by library-space. That all libraries are connected, an' you can use 'em to travel if you know how, an' you follow the rules. He didn't explain how, before you ask-said it'd be an unfair advantage."
"So..." Rose sounded perturbed. "People can get onto the TARDIS through the library, without you knowing about it? Any old time?"
"Nobody dangerous," the Doctor assured her. "At least, not dangerous to us. It takes a certain kind of person to travel in L-Space, and people who want to force their way into the TARDIS tend to not be that sort."
Rose seemed to process this. "Is The Librarian the only one who's come on the TARDIS through the library?"
"He's the only one I've ever caught. An' I'm very good at knowing if there's someone on my TARDIS who shouldn't be."
His companion relaxed a little at that. Then something seemed to occur to her. "Hang on, how d'you understand him? It can't be the TARDIS translating, 'cos I've got that too, and all I ever heard him say was 'Ook'."
"Takes a little longer for the TARDIS to work out animal translations," the Doctor said. He thought about telling her how early TARDISes, up through the Type 12, had treated any language from a civilization three or more levels less sophisticated than the Time Lords' as an animal language, meaning humans weren't considered fully sentient for approximately five hundred years of Gallifreyan history.
That was a story for another day, he decided.
Fortunately, Rose's curiosity lay elsewhere. "How much longer?" she asked. "Once the TARDIS can translate The Librarian into English 'stead of Time Lord, I can ask him about where he's from."
People, for some reason, did tend to tell Rose things more readily than they told him, but her plan still had a significant flaw. "Hard to say," he said, shrugging as if it didn't matter. "Probably around ten years or so. The Time Lords didn' really believe in rushing things."
"Really?" Rose considered for a moment, chin in hand. "All right."
"You're not gonna be still traveling with me that long," the Doctor said. He tried to say it like it didn't matter, but it did. And it was depressingly true.
"Who says?" His companion smiled teasingly. "I don't think I'll want to leave anytime soon."
He knew she was wrong, even if she meant every word of it. He could see her timeline, if he looked closely enough (and he had done, more than once, much as he tried to avoid it on the ones who traveled with him). She stuck with him for far, far longer than he would have initially guessed, for as long as she had left. But as long as she had left was far shorter than ten years.
He wouldn't tell her, not ever. So he nodded, and made himself smile back, and said "All right then", as if his permission would make any difference.
If it were about what he wanted, she would never leave.
"Talking of libraries," he said, manfully dragging the conversation in a different direction, "I've been meaning to take you to the one on Alexandria, hit its peak around 2596 during the Second Great and Bountiful Human Empire. They named the whole planet after it."
Rose listened intently, their nighttime visitor forgotten, as he described their next destination.