"Ah, yes, I was wondering if you could help me. I need to have a cat checked for listening devices."

And the emergency vet hangs up. Vets, apparently, have no Hippocratic oath. Doctors aren't allowed to hang up, even if what the person on the other end says sounds completely mad. Especially then, actually. Then you're supposed to keep them talking. I mean, if I wasn't in fact holding my potential double agent on one arm, I would have been a textbook paranoid schizophrenic case. Very irresponsible, that emergency vet. I'm not calling him again, anyway.

I set Sherly down on the desk and try and look her in the eye. "Now, I'm very aware that I'm talking to a cat, so I'll probably only be able to ask you this once before the shame gets me. But Sherly, you'd tell me if you were a spy, wouldn't you?"

Sherly is unimpressed, this much is clear. Whether with my lack of trust or my failure to provide further bacon, that's harder to tell.

Anyway, I'm talking to a cat. And I've been missing something very obvious which probably explains everything and I have nothing to actually fear. I lift the phone and dial. It makes perfect sense. The name, the address, the aura of domesticity belying the ravaged outer appearance. I don't know why I didn't see it straight away. Stood here worrying about bugs and trackers and all the while the solution was right there on the aptly-named answering machine.

"Hello?"

"Mrs Hudson, it's John."

"Oh, you're coming for dinner, aren't you? Of course you are. Can't be living on those M&S ready meals forever."

How does she know about the ready meals… She knows I can cook, so how the hell does she know about the ready meals? Never mind, more important things.

"Oh, we'll see. Mrs Hudson, might be a bit of an odd question but… have you ever owned a cat?"

Of course she has. She had a cat a while ago called Sherly but it ran away or got lost. Lived a hard life on the streets, lost half an ear, developed an insatiable lust for bacon. It explains everything, including why Sherlock never mentioned it, if she'd named it after him. Perfectly logical explanation. Of course she has.

"Ooh, no… Horrible smelly things. And the hairballs. And always dragging things to the door." Which reminds me, still have to get rid of that bird. "Eileen Donovan had one-" I don't know who Eileen Donovan is. This could go on for a while. I sit down and I keep the phone by my ear so I'll know when she's stopped talking. Mostly, though, I watch Sherly sniffing the sofa.

I don't really have the time or energy for pets. I probably won't keep her. Obviously I'll keep her here until I know who sent her, and why, but after that I'll hand her in somewhere, explain whatever sick joke the collar and tag turn out to be, and have her adopted. It's nothing against her personally, or any other cat, it's just not something I really want in my life. I've never been much of a one for pets. And these people who get all into it, who make videos and think their cat or dog or whatever is the sweetest thing alive, I wonder about people like that.

Sniffing along the base of the sofa, apparently there is some dust in the new place. Sherly pulls back, wrinkles up her scarred muzzle, bats a paw at it, then sneezes. Sneezes. Cats sneeze. I did not know this. It shakes her whole body, throws her onto her back paws. Her little tail fires out straight for a moment.

Quite against my will, I laugh.

"Well, I hardly think it's funny," Mrs Hudson says, indignant. "He went into anaphylactic shock, he nearly died."

I've missed part of the story, and she didn't see Sherly sneeze. There's no point in trying to explain, so I just apologize. She goes on as though I hadn't spoken at all. It's nice, having her talking. Having that low drone that I've just about tuned out. I've been used to having background noise, see. For a social phobic with no meaningful concept of human interaction, Sherlock never bloody shut up. I put the radio on sometimes, but it's not the same. Radio presenters occasionally change their tone of voice, and they have never, so far as I'm aware, tried to subconsciously program me to buy them cigarettes when they think I'm not really listening.

I always was listening, though. That's the really weird thing about it.

Almost a full hour later, when she knows she'll have to redial soon or they'll charge her the peak rate even on a Sunday, Mrs Hudson runs out of stories about other people's cats and says, "But no, dear, not me. Never me, not with my allergies. Why do you ask?"

Sherly has given up on exploring for now. I am lying on the sofa and she is lying on me. And she's warm and very soft and it's an unexpectedly nice feeling. "…No reason. Just thought I'd heard that story somewhere." When Sherly purrs, I can just nearly feel it. "Look, I'm not sure I'll make it for dinner."

"Yes, you will, John."

Oh. Maiden-aunt voice. Icy voice. Refuse me if you dare. I've seen genuine physical fear go through Sherlock himself at the sound of that voice.

"…'Course I will. Only joking."

"I've got a chicken on."

"Lovely."

"And some roasties."

"Can't wait."
"Four o'clock, John."

"Absolutely."

"Goodbye, John."

"'Til four, Mrs Hudson."

She hangs up. Involuntarily, I shiver. Don't get me wrong, she's lovely. Mrs Hudson, I mean. But you can't forget, the woman put up with Sherlock for years. She's got skin like a rhino and leather covers on her heart and, while I personally have never known it to happen, I don't like to think about what might happen to anyone who should cross her.

She always used to bleach the front step if Mycroft was coming. Industrial-strength, hospital level bleach. Kills all known germs and ninety-five percent of nasal lining. He'd walk over it in expensive leather shoes and smell it for days.

I'm not saying she's vicious, I'm saying she's effective as hell.

So now the plan to lie her and probably vaguely snooze in front of the television whilst puzzling over and probably stroking the grey ball of fur on my chest, that's out the window.

I pick up one of Sherly's paws and she lifts her head to look at me. How dare I disturb her. She was perfectly happy, lying there. With me. I almost feel bad. "You're going to have to move, I'm afraid."

Almost imperceptible, but there's a change in her purring. A darker note. A more dangerous one that takes me back to last night. This girl was going to take on a fox. I might have saved her then, but crossing her now still isn't a good idea.

"I know, I'm sorry, but what can I do? It's nearly one now. I have to go and get washed and dressed, and then I'm going to have to go shopping, because she didn't mention dessert, and that means it's up to me to bring it. She's an elderly woman, Sherly, I have responsibilities and I'm… I'm explaining myself to a cat, aren't I?"

I pick her up and put her on the floor. Get up and start towards the bathroom.

"Cat who's probably a double agent and all…"

A mewl. A full-blown actual miaow noise. I turn. She's standing in the middle of the floor, looking right up at me from a few feet away. Staring. Daring me to say that to her face. No. No, she's a cat, she's not doing that, cats can't do…

"Alright, I'm sorry. But I do still have to go out."

She stares a moment longer. Then lowers her head below her hackles and skulks off.

She goes beneath the desk when she's annoyed, I've noticed that already. She sulks in the dark and nudges the dead bird on the other side of the glass. And the worst thing I could possibly do is pay her any attention, so I go to the shower instead.

An hour later I'm ready to go.

I consider, briefly, putting her out and closing the window, at least until I get back.

But that feels like a betrayal. And there's really nothing to steal. I'll just leave the window open a bit for her. She can go if she wants to.

I open the door, however, just as a policeman is raising his hand to knock it.

"Oh," I say. "Sorry. Hello."

He brings up his other hand. In it, a familiar, heavy blue hardback.

"Is this your dictionary, Dr Watson?"

Oh yeah. There's a dead fox in the yard. And a mutilated bird at my window. And a battle-scarred cat under my desk.

Oh, no, my mistake; twining around my legs. Miaowing again. Drawing attention to herself and her half-ear.

…Honest, Constable, the three of them all walked into a door…