"Oook?"

Rose, who had been brought up to be polite, took the banana. She waited until the Librarian glanced away, and then slipped it into her pocket.

"Are you sure we're safe up here?" she said, for want of anything else.

"Oooqsh," the Librarian told her reassuringly, through a mouthful of fruit.

They were sitting on top of a bookshelf. It was perfectly comfortable provided she avoided looking down; although it had to be said that the view to either side was scarcely less intimidating.

The tops of the bookshelves gleamed beneath a light of indeterminate origin, and ran away into the distance until her eyes watered. She strongly suspected that there was no real horizon to this place, and that her quailing imagination was constructing one out of self-defence.

"It's a very nice library," said Rose, trying for understatement in the face of titanic splendour, "but what about yours? Aren't you worried about it?"

"Oook oook," shrugged the Librarian, an ape perfectly constructed for such a gesture. Rose stuck a finger in her ear and wiggled it about for a second, but eventually came to the conclusion that the Doctor had been right. She was beginning to understand the nuances of the ape's speech.

"Don't be like that," she said, reprovingly. "There's plenty you can do. Besides, I know how the story ends. You save the books."

"Oook?"

"I don't know how," she admitted, "but you definitely do. You'll find a way."

Rose jumped as a hand thrust itself over the top of the bookcase, waved helplessly for a second and was eventually followed by a brown pin-striped arm. The arm was joined by another, and together they heralded the rise of a dishevelled and rather irritated face.

"Two," the Doctor panted, "and a half," he gasped, "thousand feet." He grimaced, and hauled himself onto the bookshelf between them. "You do realise it took me more than half an hour to climb up here?"

"Should've used the lift, shouldn't you?" said Rose, dismissively.

"What lift?" asked the Doctor, after a pause to regroup.

"There's one about two miles down the aisle," she told him, pointing. "Between 'J' and 'Squelch'."

"Isn't Scalporian a marvellous language?" said the Doctor, beaming. "They only have a seven letter alphabet...Bang, Aaargh, Thud, Squelch, Ding, Quack and Blort. You should hear their poetry," he added, nudging Rose.

"Anyway," he went on, rooting in his pocket, "I found a shelf of these on the way up. Thought you might be interested." He extended his closed fist and dropped something into her outstretched palm.

Rose recoiled instinctively as something warm shifted against her skin, but as the Doctor drew his hand away, she stopped twitching and stared in fascination.

There was a small animal crouching on her hand. About four inches in length, it looked like a hamster, if there were a hamster somewhere in the universe that was bald, glossy and azure blue. The resemblance ended at its neck; its head more closely resembled that of a toad. She extended one finger and ran it down the line of nodules on the creature's back, which caused it to shuffle and burp contentedly.

"What is it?" she asked, not removing her eyes from those of the animal. She swore it had winked at her.

"This," said the Doctor, extending a dramatic arm, "is a library. That," he said, nodding at the gently wobbling creature, "is a book."

Rose actually felt her credulity snap.

"No it isn't," she retorted.

"Yes, it is," said the Doctor.

"No," she repeated, "it isn't." The animal burped again; without thinking, she ticked its chin lightly.

"Listen," insisted the Doctor, gently amused. "The Pellepatorians of..." he stopped, and sneezed hugely.

"Bless you," said Rose.

"Oook," said the Librarian, holding out a handkerchief.

"Thanks, but no," said the Doctor, waving away the proffered handkerchief even though it looked, admittedly, spotless. "That's the name of the Pellepatorian homeworld."

"Their planet," said Rose, scornfully, "is called 'Atchoo'?"

"Yeah. Well, it's spelled 'Waaiaatchjuuioo', but I much prefer the native pronunciation, don't you? Anyway." He pinched the bridge of his nose and tried to get back on track.

"The Pellepatorians," he explained, "are shape-shifters. They can imitate any living thing they touch, simply by sampling its genetic code. In the early days of their evolution, it started as a defence mechanism, but these days it's also how they communicate."

He paused for thought, felt in his pocket again, then pulled out a second animal and held it out on his own palm. Rose looked closely; where she'd initially thought they were identical, she could see one slight difference. The original animal's eyes were black, whereas the new one's were a deep, sombre green.

"Second edition," said the Doctor, and stowed it away again. It cheeped in his pocket.

"So," said Rose, with the air of someone picking their way through a minefield with only half the map in their hand, "what do they eat?"

"Carbon dioxide," said the Doctor, and leaned across her shoulder. For a second, she felt the warmth of his mouth an inch from her neck, and shivered deliciously. Then he turned and faced the animal instead, breathing gently on it. It pulsed happily and opened its mouth and gills, inhaling the wash of warm air.

"They breed these things as books, then?" asked Rose, when the beast in her palm had stopped vibrating.

"Yep," said the Doctor. "Just think how much information's contained in a strand of DNA. The Pellepatorians learned how to manipulate it with pinpoint accuracy. This is, as I said, a living book. They read them simply by touching them. Like so," he said, taking the creature and stroking its back thoughtfully.

"Not bad," he said eventually, wrinkling his nose and tilting his head, "but it's not going to win the Booker Prize, either. Rose?"

"Yeah?"

"I wonder if you'd mind going back to the jungle for a minute? The Librarian and I need to have a little chat, man to, er..."

"Oook," said the Librarian, pointing a warning fingernail up his nose.

"...ape!" said the Doctor. "I was going to say 'ape', all right?"

Rose sighed sarcastically and got to her feet, smoothing out the creases in her clothing with sharp, pointed strokes.

"Okay," she said, "I'll leave you to it. And here," she added, pulling out the banana and handing it to the Doctor. "You'll need this. Blimey, what a pair of m–"

"Oook?"

"Muppets," said Rose. "And you are. A right royal pair of muppets."

With that, she flounced away, heading back down the bookcase towards the lift. The Doctor watched her retreating back until he judged her out of earshot, then turned and flung a friendly arm around the Librarian's shoulders.

"All right," he said, deliberately. "What'll it take to get you out of my bloody TARDIS and back where you belong?"