Donna had just managed to close her fingers on the object under her bed when the door banged back and the Doctor barged in, shouting something. The result of this was that she recoiled, smacked her head on the underside of the bed, said an extremely impolite word and surfaced like a wounded killer whale, her face flushed.

"What?" she said, venomously.

"I said," the Doctor repeated, exasperated, "there's a cat in here somewhere."

"I know," said Donna, and held up the item she'd retrieved from under her bed. It was a bright green catnip mouse with only one ear. It was beginning to dawn on her that the mouse was also slightly damp, and she carefully adjusted her grip so that she was holding it by the tail instead.

"It's a little tabby and white cat," said the Doctor vaguely, seemingly hypnotised by the dangling toy. Donna snorted, dropped it and planted her hands on her hips.

"We're about forty million light years from the nearest galaxy," she said, slowly and carefully. "and we don't even have a cat flap, unless I've missed it. D'you want to tell me how it got in here?"

The Doctor didn't answer her immediately. He pulled out his screwdriver and tapped it thoughtfully against his cheek for a second, then dropped to his knees and aimed it under the bed. Blue light stuttered.

"Not sure," he said, eventually, still frowning, preoccupied with his task. "Normally I'd have assumed it sneaked on board when we stopped for pizza last night, but there aren't any cats on Karagon."

"So what eats the mice?" asked Donna. The Doctor looked shifty.

"Best if you don't ask," he said, "especially if you enjoyed the pizza." He paused, saw her expression and grinned. "It was a joke, okay? A joke?"

She watched the Doctor edge himself further under her bed, still scanning furiously, until he'd half disappeared. In an effort to avoid looking at his rear end, she turned towards the half open bedroom door and saw a cat. It licked its nose but made no further move.

"Doctor?" she quavered.

"What?" came the muffled, vaguely put-upon reply.

"Did you say you saw a tabby?"

"Yes," said the Doctor, and only now was there a thoughtful pause. "Why?" he asked.

"Well, there's a white one here," whispered Donna, her gaze still locked on that thirty per cent of cat that could be seen through the door.

"Don't take your eyes off it," said the Doctor quietly, emerging from under the bed and brushing smears of dust out of his hair. "See if you can get it in the room."

Nodding, Donna bent and clicked her fingers at the cat.

"Here, puss-puss," she said brightly, trying not to feel like an idiot. The cat merely looked her up and down with insolent yellow eyes that carried far more scorn than any animal ought to be able to muster, then turned and stalked away.

Donna raced for the door, yanked it back and stuck her head out into the corridor just in time to see a long tail disappearing around the corner. It was grey.

She pulled her head back into the bedroom, fixed the Doctor with an uncertain look and said, "Um..."

It was several minutes later, and they had arrived in the Garden of Artemis in pursuit of their quarry before losing it amongst the undergrowth. Donna sat on a wrought iron bench, got her breath back and watched the Doctor pace back and forth.

"What's a Schrödinger Cat?" she asked, puzzled.

"I'll explain the details later," he said, jamming his hands into his pockets and rocking back on his heels. "For now, let's just stick with the basic premise that these little buggers can travel in time and space and change colour whenever you're not looking at them, and if you feed them after midnight,, sorry, that's Mogwai, isn't it?"

Donna stared.

"Oh," added the Doctor, "and did I mention we should worry about this, because sooner or later, where you've got one, you'll get more."

It was the first time that Donna had seen the Garden of Artemis. It wasn't what she'd have expected. The garden was a riot of clashing flora, from tall plumes of pink pampas grass to hectic beds of lupins in every conceivable colour. Hanging baskets sprouted threatening clusters of dewy Venus fly traps, and there was a rather tacky stone wheelbarrow full of white geraniums. In and amongst this garish panoply were the statues that gave the garden its name; wide-shouldered, stern female archers in various states of dress and undress, most with sleek hunting dogs at their heels.

Donna was starting to get a headache just from looking at it all. She glanced up, and her jaw dropped. Above her there was a soft orange sky, shading to ochre at the horizons, with two perfect silver suns sailing in it.

"It's a projection of the dawn on Gallifrey," said the Doctor, emerging from a nearby rhododendron bush and brushing off his hands. "It's not real. It's a memory. Mine, actually. This was how the sky looked the day that..." He stopped, and looked away. Donna, not by nature one of the world's more tactful people, quite uncharacteristically bit her lip and said nothing.

"Anyway," the Doctor went on, a shade too briskly, "no cat here. I suggest we check the Archives next."

Donna trotted after him as he loped off down a rather tasteless orange gravel pathway, shoulders hunched.

"So where do these cats come from?" she asked, drawing level.

"Schrödingers are a subspecies of your basic natural Earth cat," he explained. "They were first recorded during the Middle Ages, and the theory's that in response to the pressure of natural selection at the time, some of them developed certain quantum powers like time travel, phase shift and teleportation."

"Why," said Donna, slightly out of breath from running to keep up with the Doctor's lengthy strides, "what happened in the Middle Ages?"

"Dear me, where were you during history class, Donna Noble?" he said disparagingly, looking down at her for a second. "Witch trials and inquisitions, that's what happened. Cats became hated and feared as the minions of Satan, that's what happened!"

The Doctor was so intent on his speech that Donna was the first to notice what was amiss with their surroundings. She stopped by a very familiar stone wheelbarrow full of white geraniums and cleared her throat nervously. The Doctor pulled up sharp and gave her a quizzical glance.

"I'm not an expert," she said carefully, "but weren't we here just here a minute ago?"

"Yeah," he said, distractedly, running hand through his hair. "I think..."

Whatever he had been about to say was lost in a violent cacophony of shrieks. A small cat darted out of a shrubbery, eyes wild with terror, and zig-zagged across the clearing with its ears laid flat. Donna had barely had time to register this when a rolling tide of felinity crashed out of the same shrubbery and skimmed off in pursuit.

The Doctor, caught in this unexpected rip tide of fur, pranced around on one leg for several precarious seconds while grabbing for equilibrium with both hands, and managed to regain his composure just as the last stragglers pelted past his ankles. This achieved, he glared at Donna – who was laughing so hard that she had to hang onto a nearby statue – and sprinted after the cats.

He was back just a few seconds later. He tripped over the wheelbarrow, struggled for balance once more and, this time, failed spectacularly. Donna helped him clamber out of a tangle of squashed lupins and picked a petal off his lapel.

"Did you see them come through here?" he asked, irritably. Donna shook her head.

"No, just you," she said. "How did you manage to come from that direction?"

He didn't respond immediately, but when he did, he sounded annoyed.

"Using pockets is naughty," he said, severely. "What did I tell you last time? You can't keep them, okay? Now let us out! Yes?" he said, this last to Donna, who had tugged at his sleeve.

"Who are you talking to?" she asked.

"The TARDIS, of course," he said, as if this fact should have been apparent. "She wants to keep the cats. Every time an animal gets in here, she wants to keep it. I've told her time and again she can't have a pet."

"Why not?" said Donna, frowning. The Doctor withered her expression with one of his own.

"Because," he said with badly forced patience, "if I let her keep every animal that's ever managed to get in here, she would now be the proud owner of eighteen dogs, ten cats, four reptiles, forty-four assorted rodents, eleven primates and seven thousand, six hundred and nine variations on the theme of insect. Oh," he added sourly, "and one velociraptor."

"Um. Well," said Donna, after a decent interval, "that's me told...but it still doesn't explain how you're running around in circles. What's a pocket?"

The Doctor explained about four dimensional relativity and how the TARDIS could shape the continuum with impunity. He spoke at length about pinched spacetime and subdimensional curvature. He waxed rhapsodic on the subject of quantitative phase adjustment. He would have elaborated upon the finer points of non-Euclidean tangential geometry but for the fact that at that point, Donna threatened to punch him.

"It's a bubble in space," he told her, lamely. "It's perfectly self-contained, just very small. If you go more than twenty metres in any direction at all, you'll end up right back where you started."

"That's impossible," said Donna. The Doctor shot her a brief but meaningful glance that spoke volumes about his feelings regarding the word 'impossible', and then regrouped.

"Not for the TARDIS," he said, firmly. "She does with time and space what you used to do with Lego as a kid, believe me."

"You mean she makes half a house with no windows or doors, and then gives up and starts crying because the plastic tree's broken and all the people look scary because they've got no hair?"

There was a very long, luxuriant silence; the kind of silence that is very rare in mortal realms, and is only rendered possible because two people are staring at one another in an unbreakable deadlock of bewilderment. The entire universe shuffled its feet awkwardly.

"Right," said the Doctor, carefully. "The Archives, then?" He made as if to set off, then paused and glared meaningfully at the sky. Donna felt a very faint sensation of pressure on her middle ear, and then it was as if the garden had twisted around her. By the time she managed to shake off the peculiar aftershock of this, the Doctor had crunched off down the path.