Just a thought that came to me one day and wanted to be written up. Usual disclaimers apply - I have no rights over John, Sherlock or Mrs H. Huge thanks to liverdoc for medical and feline knowledge, and as always to my lovely beta RedSkyAtNight. Hope you enjoy!

"John, have you thrown my tissue samples away?" Sherlock's irritated voice floated through the hot, still air of the flat at 221B.

John looked up from the Lancet article on which he was unsuccessfully trying to concentrate. "What? You mean those dishes of putrefying stuff on the windowsill? No, of course I haven't. What's the matter with them?"

"They've gone. Again. That's the second time today." Sherlock sounded increasingly disgruntled. John got up and crossed the living room to where his friend was regarding three empty Petri dishes with an expression of distaste. They bore barely a trace of having ever held organic material.

"What were they, anyway?" John wasn't particularly interested, but any distraction was welcome in this stultifying heat.

"I'm doing an experiment on the rates of desiccation of flesh in warm air. There are a number of cases in which it could be highly relevant. That murder from last month that Lestrade still won't let me drop, for instance."

"Oh." John took a moment to process, then frowned slightly. "You mean that was human flesh in there?" His tone registered interest rather than surprise.

"No. Molly is on holiday" – Sherlock's tone made it sound as though Molly had no business to be doing anything so inconvenient – "and the rest of the staff at Barts have been ridiculously unhelpful about allowing me access to sample material, so I had to do the best I could in the circumstances. It was pork."

"Ah. Well, whatever it was, I haven't touched it. And Mrs Hudson hasn't been in…" John looked at the window, opened to its widest in a vain attempt to catch any breath of air that might cool the flat on this sweltering July afternoon. Across the rooftops that spread below, the dull roar of traffic and general London activity floated in. "Could something have come through the window and taken them? A crow perhaps, or, oh, I dunno, rats? It's not too far up from Mrs Hudson's kitchen roof."

"Possibly. No dust to show footprints, but –" Sherlock picked up one of the dishes and examined it more closely – "whatever it was has made quite a thorough job of licking it clean. Not a bird then, I surmise."

"True," John replied flatly. "I suppose you could shut the window, but then, I might have to kill you. If we didn't die of asphyxiation first."

Sherlock ignored the attempt at humour – ignored, or, perhaps, didn't notice, John thought. "No, I couldn't. The experiment requires the passage of warm air over the samples." He emitted a sigh of boredom and exasperation. "I suppose I shall have to start again."

"Mmmm." John couldn't raise the energy for any more. He returned to his armchair and tried once again to interest himself in The efficacy and safety of canagliflozin versus glimepiride in patients with type 2 diabetes inadequately controlled with metformin, while the sound of clattering and indignant muttering from the kitchen indicated that Sherlock was cleaning the Petri dishes in readiness for another attempt.

By the evening, the sound of the traffic had died down to a muted rumble and the temperature in the flat had dropped to more bearable levels, although there was still barely a breath of wind to stir the curtains by the open window. Dusk had come and the flat was in shadow, save for a pool of light from a lamp which illuminated Sherlock sprawled on the sofa in his dressing-gown, engrossed in trying to decipher an ancient Sumerian code. On the other side of the room John lay in his armchair, the Lancet open across his chest, eyes closed, mouth open, snoring slightly.

After a while John twitched and blinked, slowly returning to consciousness. As he opened his eyes for a second time, a slight movement in his peripheral vision made him turn and look at the window. After a moment he said quietly, "Sherlock, I think we've found your culprit."

"What?" Sherlock looked up, frowning, and John nodded towards the windowsill. Sherlock looked over to see a large tabby cat helping itself to the samples in the Petri dishes with no sign of unease.

"Oh! Get away with you! Go on, shoo!" Sherlock leapt to his feet and across the room, flapping his arms at the cat but refraining from actually hitting it. The cat cowered away from his hand but then seemed to recognise that it was not in any real danger; it grabbed the last morsel of meat from the dish and then retreated only to the outside window ledge, where it sat up and began to clean its whiskers with one paw.

"Go away, you wretched animal!" Sherlock flapped at it again but the cat took no notice. Slightly amused, John got up to take a closer look. "It seems to have your measure, Sherlock…oh." His voice changed, registering concern as he got a better view. "Look, it's hurt, poor thing."

It was difficult to make out in the dusk, but under the animal's chin hung a flap of skin, and dark areas of dried blood denoted further cuts around the back of its neck and on one ear. All the injuries were in places where the cat was unable to lick itself; it was doing its best by means of washing with a paw, but as John bent to get a closer look, he caught the unmistakeable whiff of infection. Professional curiosity roused, he reached out to the cat. Perhaps because he stroked it first, the cat made no objection as John scooped it up and carried it into the flat, holding it under the lamp to inspect it further.

"Oh, that's a nasty cut… infected under the chin, there." As he held the animal he became aware of something else. "Awfully thin, too, I can feel all its ribs. Must be a stray… I suppose that explains why it was so keen to eat your samples." He turned the cat's head to examine the rest of the cuts. "Wonder how you did this to yourself, poor fella?"

Sherlock sighed irritably, crossed to the lamp and, for the first time, gave his proper attention to the cat. After a few seconds he began to speak in the jerky, rapid-fire manner that John recognised as his "exposition" voice.

"Mature cat, quite large, obviously had a reasonable start in life but has been living rough for quite some time to have lost that much weight. No visible injuries to any other parts of the body so unlikely to have been hit by a car, jaw not broken so injuries not caused by an impact like a fall or a kick. So, just the cuts from a sharp object, what could that be, maybe a tin, yes, highly likely if it was looking for food, going through the bins, quite a large tin with a bit of something meaty left in the bottom, dog food perhaps or a catering-size tin of stew or similar, now, if that tin had been opened nearly all the way but not quite, scraped out and then the lid pushed back in, cat comes along, shoves its head inside to reach what's left, when it tries to pull out the bent-in lid catches under its chin, cutting in, the harder the cat pulls, the more it digs in, the edge of the tin causes the cuts round its neck, look, even where the skin isn't cut there, the fur is, yes, that's the most likely explanation. Must have got free eventually, but left with injuries, couldn't reach them to wash them properly…" His tone of voice changed abruptly. "John, what are you doing?"

This came as Sherlock watched John, cat still firmly tucked under one arm, retrieve his doctor's bag, go into the kitchen and turn the light on. Once there, he plonked the cat on the table and began to search through the bag with one hand. "Sherlock, could you come here and hold him still for a moment, please?"

"What? Why? You're treating it? What for?" Sherlock hadn't moved from his place in the doorway.

"Yes, of course I am, we can't leave him like this, poor fella." John's medical instincts made it impossible for him to leave an injured creature untreated, even if it was only a cat. "Look, just hold him for me, will you?"

"But it's…"

"Him. It's a him. I can help him – look…" John finished awkwardly fishing in the bag one-handed and brandished a pair of surgical scissors. "If you just hold him still, I'll cut that loose skin off and clean the wound, that's what he needs."

"Oh, all right." Sherlock sounded unconvinced in the extreme, but crossed to the table and held the cat firmly while John trimmed away the flap of skin and then gently cleaned the wound with a disinfectant wipe. Next he applied antiseptic cream to all of the creature's cuts, before searching in the bag once more.

"Could really do with an oral antibiotic to knock that infection on the head… these are for humans but pretty generic, I reckon a half dose would be OK for a cat… Of course, it'll be getting him to take it that's the tricky bit…" John didn't really know whether he was talking to himself or to Sherlock, but he located a small bottle of pills, opened it and broke one in half. "Still, if he's hungry enough…" He went to the fridge, located a packet of ham and carefully wrapped a piece around the tablet. "Here, try that, fella…" Luckily the cat wolfed down the ham, tablet and all, and looked hopefully for more.

"You need feeding up a bit, don't you?" John addressed the cat directly, causing Sherlock to roll his eyes in disgust. "Better not give you more ham, too rich, let's see… how about some tuna?" John found a tin in the cupboard, opened it and forked half of it onto a saucer. The cat tucked in with gusto.

"John, we're not keeping it."

"No, but, we can't leave him like this, can we?" John looked faintly embarrassed to be seen to be lavishing such care on a stray animal, but reminded himself that he had no need to be – it was a matter of common humanity. "We'll just keep an eye on him for a few days until the wound heals, that's all." He began to clear up the medical debris from the table.

"Keep an eye on h – it? It might not even stay here. You've already agreed it's too hot to close the windows, it could be gone by morning."

"He could, but while he knows there's food here, there's a good chance he'll keep coming back. Soon got wise to your samples appearing, didn't he?"

"Hmph." Sherlock sounded annoyed to be reminded about the fate of his experiment. He turned and silently stalked back to the sofa to take up his book. John busied himself with tidying and what needed washing up. When he returned to the living-room five minutes later, he found the cat curled up in his armchair, purring contentedly. Resignedly, he left it there and headed to bed.

In the matter of animals, John Watson had always thought of himself as a dog man, the sort of man who, if he had a pet, would choose a no-nonsense kind of animal like an Airedale or a bull terrier. And yet he had been as kind as anyone to the stray cats that had from time to time wandered into Camp Bastion, feeding and looking out for them as the other soldiers did, and treating their minor ailments when he could. And he had grown to appreciate their company. There was something about being far from home and living with a constant, if varying, sense of danger that made a man grateful for the mute, non-judgmental company that an animal could provide.

He remembered with particular fondness one small tortoiseshell cat who had often curled up on the foot of his camp bed at night. It had been comforting sometimes, when woken by the sound of RPGs flying overhead, to be conscious of her soft, gentle presence. And so that night, when distant thunder half-woke him in a haze of confused memories, he was glad to reach out, find a warm, purring bundle curled up by his legs, and sleep again.

Sherlock got up the next morning to find a note on the kitchen table. It read "Gone to work. Cat not here this morning – if he comes back could you give him some more food and another half of these pills? J." It was weighted down with a small bottle.

Sherlock read the note, frowned at it for a moment, and then completely forgot about it.

An hour or so later he was completely engrossed in studying a pathogen sample under the microscope when a firm nudge against his hand made him knock the slide, with its sample, flying across the table. He barely had time to register what had caused it when his hand received another furry head-butt, this time accompanied by a loud yowl. Cursing under his breath, he stood up and surveyed the damage, in the middle of which stood the cat. It returned his stare and, miaowing again, walked towards him across the table, tail in the air.

Remembering John's note, Sherlock took a half pill from the bottle, crossed to the fridge and wrapped the medication in a slice of ham, just as John had done the previous night. He then proffered the package to the cat, which took it daintily from his fingers and consumed the ham carefully. The pill clattered onto the table-top, untouched.

Scowling like a thunderstorm, Sherlock picked it up and considered repeating the procedure, but instead emptied the other half of the tuna onto a saucer and concealed the pill in the middle of it. He placed it on the floor, where the cat leapt down and immediately began eating, purring as it did so. Sherlock retrieved his slide and put it under the microscope again.

Thirty seconds later he looked up to see the cat strolling away from the saucer, licking its lips appreciatively. His feeling of satisfaction evaporated when he glanced at the saucer to find it licked practically clean, save for a tiny scrap in the middle on which sat, with a certain inevitability, the uneaten half-pill.

With an inarticulate yell Sherlock lunged across the room and grabbed the cat, secured it under one arm, and then retrieved the pill. Next he tried one-handedly to manoeuvre the writhing animal into a belly-up position, while with his other hand attempting to forcing open its jaws, insert the pill and clamp them together again quickly. The whole sequence turned into a yowling, spitting tornado of fur and curses which culminated in the cat erupting out of his grasp and streaking into the next room. Behind it, the pill bounced harmlessly onto the floor.

Sherlock put his bleeding index finger into his mouth and sucked thoughtfully, at the same time inspecting the lacerations on the knuckles of his other hand. He followed the cat into the living room and found that it had retreated to the furthest corner of the highest bookshelf where it crouched, staring at him, lashing its tail, just out of reach.

Muttering curses, Sherlock served out another saucer of tuna and put it on the floor below the bookcase, then retreated a little distance to see if this could tempt the cat down. Appetite sated by this time, it merely stared at him. A saucer of milk placed next to the tuna similarly had no discernible effect.

Sherlock glared at the cat, thinking. What did people do to attract cats? They made a sort of wheedling noise, didn't they? He essayed a sound at the back of his throat – oh God, what was he doing? He sounded like Molly. The whole thing made him feel completely stupid, and the cat's expression seemed to indicate that it concurred with his opinion. Completely disinclined to start climbing on furniture in order to reach it, he went back to the kitchen and ignored it for the rest of the day.

"Evening!" John's voice rang through the flat, making Sherlock lift his head from the microscope. John bustled into the kitchen bearing several Sainsbury's carrier bags. He dumped them on the worktop and began to unload them, stowing groceries away in fridge and cupboards. "I got bread and milk, oh, and some pouches of cat food, thought they'd be the best thing for him – has he been back?" John turned to look at his flat-mate.

"Yes."

Oh, good, did you give him a tablet?"

"No."

Not for the first time, John wished that Sherlock's conversation could strike more of a happy medium between monosyllabic and non-stop. Knowing that asking "Why not?" would be unproductive, he said nothing, although a slight smirk passed across his face as he took in his friend's scratched hands and the traces of fur on his shirt. "Where is he now, then?" John's question was rendered obsolete by a heavy thump from the other room as the cat landed down from the shelf. Moments later it wandered into the kitchen and began to weave around John's legs, purring loudly.

"Hello, fella, how are you doing today?" John bent and picked up the cat as he addressed it. "Oh yes, those cuts are looking better already. Well, let's give you another pill, make sure you carry on improving."

Sherlock watched impassively as John went through the same procedure with the pill and the ham as he had himself tried, with the same result. His attempt to hide the pill in a saucer-full of cat food also failed, the pill again being left in isolation with the food eaten neatly from around it. John allowed himself a small frown.

"Looks like we're going to have to do this the hard way... can you hold him for me, Sherlock?"

"I'll get fur all over my shirt."

"You've already got fur all over your shirt. Just hold him, will you?" John picked the cat up and plonked it in Sherlock's arms, then bent for the pill. Between two of them it was a relatively simple operation to hold the cat securely, open its mouth and push the pill to the back, where it was swallowed. Seemingly realising that the odds had increased against it, the cat put up only a token resistance.

"Looks like we'll have to do it like that in future," John commented, washing his hands.

""In future"? I've told you, we're not keeping it."

"Yes, I know, just till those cuts are healed up."

Sherlock declined to comment further.

After dinner they sat in the muggy living-room, dusk falling again. John tried to find something entertainingly mindless on the TV, but Casualty never failed to irritate him, Sherlock answered all the questions on University Challenge before he had a chance to, and both of them considered Masterchef to be beyond the pale. John yawned, flicked off the TV with the remote and tried to find the motivation for once again doing battle with the Lancet article.

Sherlock meanwhile was seated at the coffee table, under the pooled light from the standard lamp. On the table lay a large number of strips of paper, the remnants of a number of shredded documents which were apparently relevant to his latest case. Totally absorbed in his work, he was painstakingly re-assembling them, strip by strip, assessing each one in turn and arranging them carefully like the pieces of some fiendish jigsaw puzzle.

The cat wandered through from the kitchen and John tried to attract its attention, making chirruping noises and patting his lap encouragingly. It stared at him contemptuously for a moment, turned, and then, in a moment which he could watch unfolding before him but was powerless to stop, it leapt onto the coffee table. Shreds of paper scattered in every direction as it butted against Sherlock's hands with every appearance of affection. Exasperated beyond measure, Sherlock leapt to his feet and launched into a tirade.

"Why do people like cats? Why? They are the most irritating, provoking, infuriating animals on the planet! When you want them they ignore you, when you're in the middle of something they come and disrupt you. You try to help them, they're completely uncooperative and ungrateful… Honestly, I've never known anything so contrary, so perverse, so – so – utterly capricious – what are you smirking at?" His tone changed abruptly as he registered John's expression.

"Nothing." John tried and failed to look innocent.

"What?" Sherlock's voice dropped deeper as he demanded an answer.

"No, well, it's just that, you've never tried to live with you, have you?" John grinned up at him cheerfully. "Some of those characteristics sound rather familiar – you know, the ungratefulness, the perversity… Just an everyday experience for people that know you, really."

"Don't be ridiculous, John." Sherlock glowered and stalked off to bed.

The cat hung around the flat for three more days, during which time they managed between them to dose it regularly with medication, and its injured areas improved significantly. On the fourth day, however, it disappeared, and although they continued to leave the windows open, they saw no further trace of it. John told himself that it was only to be expected: it was a stray cat, it was used to the itinerant lifestyle and was bound to move on. That didn't stop him occasionally reaching out for it in his sleep, though, nor during his waking hours hoping that it was managing to find food and shelter. Sherlock didn't give it another thought.

A week later, as John opened the door from the street on his way home from work, he was met by Mrs Hudson sticking her head out from her ground-floor flat. "Oh, hello, dear, I was hoping I'd catch you. A package came for you earlier and I took it in – hold on a moment while I go and get it…" She ducked back into the flat and John came to her door to wait.

Looking into Mrs Hudson's cosy living-room, he was taken aback to see the tabby cat, stretched out comfortably on a blanket in an armchair. A quick scan of the room revealed bowls of milk and minced chicken on the floor, and a soft brush lying on the arm of the chair. Clearly the cat felt perfectly relaxed.

"Here you are, dear…" Mrs Hudson returned and, following John's open-mouthed gaze, turned towards the cat. "Oh yes, that's Mister Tribbles, well, that's what I call him anyway. He turned up about a week ago, seems to have made himself quite at home. I must admit I've been spoiling him a bit…Well, it's nice to have an animal about the place, isn't it?" She smiled as she handed him the parcel.

John looked back at the cat, which opened its eyes and gave him a steady stare. It seemed to say that it was aware it had been given a ridiculous name, but that on the whole, it considered that to be a reasonable trade-off for the very comfortable surroundings which it was now enjoying. John couldn't help but agree with it. Thanking Mrs Hudson for the parcel, he turned and began to climb the stairs, his mind set at rest.