The console room was full of fish.

It seemed to the Doctor that this was a slightly flippant observation, since it was by no means an hyperbole; the spaces beneath the console were crammed with salmon, the rafters festooned with sardines, the decks laid with drifts of slippery whitebait and every crevice in the walls packed with glossy pink tuna steaks. He saw his work, and saw that it was good.

Brushing his hands gleefully, he hunkered down behind a pile of rainbow trout and, trying to ignore the distressing odour about the place, settled down to wait.

It had not been more than a few minutes before one of the piscine mounds near the far door shifted and dislodged a black cat, which was already gripping a small halibut in its jaws with the expression of an animal that means to keep on gripping. It surveyed the room with disdain, then settled beneath a nearby panel and began to eat.

Minutes passed, and more and more cats filtered into the console room. One or two minor fights broke out, but since there were several hundred fish for each cat present, they soon dissolved into embarrassed preening and half hearted hisses.

The Doctor peered through a gap between two dorsal fins, narrowed his eyes, and counted. When he was satisfied, he reached out, without looking, and groped for the button that would seal the doors.

Before he could do so, however, Donna stepped through into ankle-deep flounder. She looked down and then back up again with a nonchalance that only eight months of travelling with the Doctor could bestow, and then frowned at him.

"You can come out," she said, quietly. "I know why they're here, and I know how to get them to go away."

The Doctor was so surprised by this that he bounced up from behind the wall of fish. He opened his mouth for a second, looked thoughtful, then closed it again and rooted in his pocket. Only after he had removed the sardine did he refocus on Donna and, more importantly, the drowsy bundle in her arms. He frowned quizzically.

"Isn't that..."

"Yes," said Donna, hugging the cat to her. "It's the first one. This is why the others showed. She's in season."

"How d'you know that?" asked the Doctor, although Donna watched a subtle expression flow across his features that suggested that he already knew the answer.

"I know because this is Sweetheart, and she's my cat."

The Doctor, to give him his due, simply nodded sagely.

"How long has she been missing?" he said, not taking his eyes off the contented animal, which shuffled in Donna's arms and purred like a distant chainsaw.

"Thirty years or so," said Donna, with a small shrug.

"Only from your perspective," said the Doctor absently, now almost totally lost in thought. "Give her here. I want to find out what happened."

Reluctantly, Donna handed over her charge, and the Doctor sat down on a handy step with the cat sagging happily across his lap. He chuckled softly, then took its head between his hands and concentrated. His breathing slowed and roughened, quickly becoming hoarse and soporific.

Donna had taken up station behind his shoulder, and now she watched as the cat's eyes widened, pupils dilating until the animal's soft yellow irises were almost lost to view. The steady purr also deepened, becoming throaty, and slowed until its rhythm matched that of the Doctor's breathing.

"Okay," said the Doctor, his voice jagged, and sounding as if he were speaking through several layers of cloth. "She was in the garden...she heard you calling, but all of a sudden, there were other cats."

"Males?" said Donna, quietly

"Yes," breathed the Doctor, still lost in the cat's glossy eyes. "They chased her. She couldn't find anywhere to hide. She panicked. Did the only thing she could. She slipped through spacetime."

"She's a Schrödinger Cat?"

"Half Schrödinger Cat," said the Doctor, as the cat's ears flickered. "A dormant gene; it only came into play when she first came into season. No wonder she didn't know how to handle it..." he hesitated, and all of a sudden, his back stiffened. "Okay, that's not good. Looks like she ended up in Australia first of all. She's wondering what to do about the enormous rats.

"She's jumping again, and this time...what's that? It's dark. Looks like a rhino. Is she in Africa? I...ah, no," he muttered, grimly, "it was the horn that confused me. It's a triceratops."

Donna gaped, and drew breath, but the Doctor was already continuing.

"She's in the void now. Trying to pick an exit, but there are too many. She's looking for a trace of you...there!" he cried, suddenly all exuberance, but still not breaking eye contact with the cat. "She found you. We were passing by. She didn't know how much time had passed for you, though. She jumped, but while the door was open..."

The Doctor exhaled loudly and harshly and released his delicate grip on the cat. Free of the trance, it nuzzled his hand lovingly, whiskers pricking and flexing with delightful abandon.

"We can take her home," said the Doctor, craning his neck to catch Donna's eye at last, "but it's up to you. If you want to keep her, we can do that, too."

There was a long, and very long pause. Donna felt the universe expand just a little further towards infinity, and it tickled the back of her neck, as if she felt she were being watched. Finally, she shook her head.

"No," she said, firmly. "I miss her now, but I missed her even more when I was little. Let's take her home."

The Doctor nodded and broke his gaze, bouncing to his feet, running both hands through his hair until it looked even madder than usual.

"The seventeenth of August, 1975, then," he said, brightly. Donna recoiled as if she'd been smacked.

"How do you know that?" she demanded.

"I don't," said the Doctor, already preparing the console. "She does," he said, and angled his head at the cat. Then, without waiting for any further response, he slapped at a lever, and the TARDIS twisted into a handy crack in the fabric of the universe.

When her eyes had stopped rotating in their sockets, Donna shook her head and saw, first of all, that the fish had gone. Even the smell of fish had gone, which she considered was a minor miracle even for the TARDIS; it was one of those smells which somehow contrived to crawl into the nostrils and camp out there until forcibly ejected.

The second thing she noticed was that the cats were clustered around the door, staring up expectantly, eyes wide. The Doctor picked his way through the furry minefield and opened the doors. There was nothing out there but blackness, although a painful ache in the back of her retinas told Donna that this was not simply due to a lack of light, but due to a lack of reality.

"Everybody out," said the Doctor, not menacingly, but with an edge to his voice that hinted at future menace, depending on circumstance. The cats rose and, in pairs, trotted out into the nothing. Donna picked up Sweetheart once more and ran distracted fingers through the cat's fur, but otherwise watched in silence until the last wavering tail had vanished into the sucking blackness outside the TARDIS.

The Doctor shut the door behind them, and heaved a hearty sigh of relief. Then, hand still on the latch, he counted to fifteen under his breath while Donna watched, baffled beyond endurance. She was just about to interrupt this bizarre deliberation when the Doctor raised a cautionary hand and dragged the door back once more.

It was the scent that hit Donna first, carried in through the door on a warm, pleasant breeze. It hadn't been a part of her life in many years, but the scent brought the memory straight back at once. There had been a magnolia tree in their garden when she was small, and she could smell it once more. The sensation ran down her spine and into her feet, and she took a step forward.

The door slammed. The Doctor, who had been standing a little too close, leapt back a full six inches. His eyebrows knotted, and he craned his neck at the ceiling.

"You can't keep her," he said, firmly, and wagged a finger for emphasis. "She's not yours, and besides, she..."

"Doctor," interrupted Donna, softly, "it's all right. I'll handle this." She took an even breath, then looked up.

"I love her too," she said, eventually, despite addressing her voice to what – had she not known better – would have felt like thin air. However, she sensed a tiny change in the air pressure, and understood that the TARDIS was at least paying attention.

"I want to keep her too," she went on, "but it's not fair." Donna paused and cast a glance at the door, but it remained closed.

"You don't have to do it because you were told," she said, cocking her head at the Doctor, "but you can always do it because it's the right thing to do. How's that?"

No response. If anything, the door seemed to huddle against the frame.

At that moment, the cat wriggled insistently in Donna's arms, snaked out from under her elbow and dropped to the floor on four soundless feet. It curled its tail questioningly, first one way and then the other, and looked back over its shoulder at Donna for one long, one very, very long moment in time. Donna rubbed at her eyes with the back of her hand, but nodded.

"See you soon," she whispered.

The cat paced across the floor and reared up on its hind legs, placing pink pads against the door of the TARDIS. It tilted its head and gave a soft, querulous, "meep", and at this, the door finally swung back in silence.

The breeze rushed in once more, this time carrying with it not only the heady scent of the magnolia, but also the soft refrain of a young girl's voice; not afraid, but sounding a little concerned.

"Sweetheart? Where are you? Come here, girl."

Donna opened her mouth and started to move, but the Doctor frowned at her and laid a finger across his lips.

"Come on, girl," said the distant voice once more, "time for dinner."

The cat looked back at Donna once more, but this time there was no sad query in its gaze. It blinked carefully, turned about once more, and slipped out into the hazy summer night.

The Doctor closed the door behind it and exhaled gratefully.

"That's all sorted, then," he said, cheerfully. "Where to now?" Donna, returning to reality with a thump, her eyes rimmed with pink, gawped at him in frank disbelief.

"Sometimes," she said, her voice winding up, "you can be the most arrogant, insensitive, egotistical..."

"Stop, you're making me blush, said the Doctor, ramming his hands in his pockets and grinning.

"So where are we going now?"

"Sorry, isn't that what I just asked you?"

"I don't know. Wherever. Atlantis?"

"You didn't pack any wellies."

"I mean before it started sinking, you dumbo."

"Oh good grief, stop being a pain and let's just hit Random, okay?"

He pressed the button.