33%

It had been bad enough when there had been bits of body hanging round. Severed hands evicting the milk from the fridge, or eyeballs bobbing in the bathroom sink; these had not been ideal living conditions. The whole body, John thinks, eyeing it as it reclines in the bath that he is never going to use again, ever, is too much. It is altogether Too Much Body.

"Flowers are traditional," he informs Sherlock, returning to the living room. Sherlock does not honour this with his acknowledgement, so John adds, "I think we have to tell Mrs. Hudson if we start sub-letting."

"We're not sub-letting," says Sherlock. "We're flat-sharing. It's temporary."

"How temporary?"

Sherlock pretends to be deeply involved in whatever he's doing (which appears to be piling old books to shoulder-height on the coffee table, albeit with a very dedicated look on his face. This is clearly not just book-piling, this is book-piling For Science).

"I want to know how temporary," says John.

"I'll take it back at the end of the week," Sherlock says delicately, as if he is doing John a big favour with this.

"At the end of the – no you won't, Sherlock, you'll take it back today. Did you ask permission to borrow that body, dare I ask?"

"I couldn't ask permission," Sherlock points out. "It was already dead. They're not typically all that talkative once they're dead, though I encourage you to experiment with the corpse that I have conveniently provided."

"Brilliant," says John. "Thank you. Thanks, that's definitely what I meant." He weighs up the likely length and intensity of Sherlock's inevitable sulk, and then adds, "End of the day; I mean it. That's my limit."

Sherlock perks up. "Your limit for the proportion of dead people in 221b is approximately 33% for around twelve hours?" He stands up and knocks his books over, but he doesn't seem to care. John's suspicion that he was simply piling things up to look busy is all but confirmed. "Is that percentage your ceiling, or could I have, say, four dead people – 66% – for six hours?"

"I don't want to be outnumbered by the dead people," says John.

"50% dead people," Sherlock proposes, "and no tall ones."

"We're not talking about buying a puppy, Sherlock," John huffs, retreating into the kitchen to put the kettle on, "I'm not going to let you pick out a new one. You have one dead body, for one day. That's it." He bangs two mugs down on the draining board to underline this ruling, and tries not to imagine Sherlock climbing in a cab with a corpse slung over his shoulder. There is a loud clattering sound from the living room, which he pointedly ignores.

"And you have to clean the bath afterwards, too," he says after a minute of this, although he is under no illusion that Sherlock is paying attention. "What if I'd wanted to use it?"

"You weren't in it," says Sherlock, from the living room. He is sounding rather strained, for some reason. John pokes at a half-drowned teabag with a spoon.

"I didn't know I had to make a reservation, that's not how it usually – have you ever even taken a bath?" He glances up, but there is no sign of his wayward flatmate; John sighs and sticks his head out of the door, a striped mug of tea in each hand. "What are you doing n–?"

He swallows the end of the question, eyes wide. It is very obvious what Sherlock is doing now.

Sherlock is fighting a zombie.

To his credit, John does not drop the tea.

"You've made a zombie," he says, because he just wants to be sure.

"I haven't made a zombie," Sherlock says scathingly, which would be more convincing if there were not a zombie in the flat. He is gripping the struggling corpse by the wrist – the corpse that, ten minutes ago, had been lying in peaceful repose in their bathroom – as it claws wildly at him. One eyelid has stuck slightly, and its skin is sort of grey around the edges; this, and the lopsided mouth that seems to be straining towards Sherlock's shoulder, is what convinces John that their new companion has not simply made a miraculous recovery. "I appear to have accidentally borrowed a zombie. I'm not – at all –" he makes one great effort and there is a horrible snapping noise as one of the zombie's arms breaks and Sherlock jumps backwards away from it, " – responsible!"

The zombie moans wordlessly, lunging after him; John abandons the tea and snatches up one of Sherlock's big books about poison, holding it in front of him like a weapon, something a bit like hysteria rising up at the back of his mouth. "You said they weren't talkative once they were dead!" he accuses Sherlock.

"Well, technically it isn't talking," Sherlock points out, dodging around the coffee table as the zombie snatches for him. He seizes the standing lamp and brandishes it threateningly, attempting to push back the newly reanimated corpse; John holds his book at the ready and stares wildly at Sherlock, still not really feeling that he has received a legitimate explanation for this.

"Did you know this was going to happen?" he demands, and Sherlock looks enormously insulted.

"If I'd known this would happen, John," he says, "do you really think I'd have kept it in the bath? Clearly it's infected with – some sort of virus; it's still entirely dead," he had checked its pulse, John realises, when gripping it by the wrist, "but it's functioning on an automatic level – the flesh is being controlled by the virus –"

"Stop looking so bloody gleeful about it," says John, more angrily than he expected, "and let me know if it's brain dead or if I can bash it with something."

The zombie manages to evade Sherlock's lamp-wielding at this point, and darts forward again, mouth still hanging wide open; Sherlock throws the lamp aside and leaps onto the sofa, jumping across to the opposite side of the room in a whirl of dressing gown. "You may," he says, although he hasn't stopped looking gleeful, "bash it."

John steps forward, book raised high, and the zombie turns its attention to him and rushes to close the gap. Its left arm is still hanging uselessly at its side where Sherlock has snapped it, but it gives no indication whatsoever that it has even noticed this, intent as it seems to be on gnawing holes in its two hosts.

He backhands the book perfectly into the zombie's face, knocking it right off its feet and onto the floor, with a horribly solid thump.

The zombie lurches back to its feet, undeterred.

"Bollocks," says John. "We can't knock out the bloody thing."

"Controlled by a virus!" crows Sherlock. "No consciousness or pain recognition remaining!"

"I'm going to hit you with this book next," John warns him. "And then we'll see if you have any pain recognition, alright?"

Sherlock shuts up, but he does dash around the armchair and start rummaging around in the clutter while John faces down the persistent zombie; and if he has got distracted by something else, John thinks, giving his opponent another good whack, John will kill him.

When Sherlock straightens up, he is wielding a sword.

"A foil," he corrects John, before the latter has even said anything.

"Excellent," says John, although he doesn't think the sarcasm quite comes off. "Are zombies afraid of foils?"

"That's not the point," says Sherlock, bounding forward by way of a zombie-distraction; the thing chases after him, but he actually is quite good at fending off its attempted strikes with his damn swor- foil. He is also very obviously aware of this, and of how gratifyingly dramatic the swish of his dressing gown is. John is not sure he is taking the zombie threat entirely seriously.

"Get it into the bathroom," he says, "and we'll shut it in there until we can get, I don't know, Mycroft or someone to get rid of it."

Sherlock stops mid-parry, looking outraged, and almost gets bitten by the zombie. "We're not giving it to Mycroft!"

"No," says John, opening the bathroom door as wide as he can. "We're not giving it to Mycroft. Mycroft is taking it off our hands. On account of it being, you know, homicidal."

"You didn't like it even when it wasn't homicidal," Sherlock says, looking wronged.

"I maintain that that wasn't unreasonable of me," says John.

"And you said I could have it for the rest of the day."

"That was before it started causing trouble."

"That was before it got interesting," Sherlock moans. Despite his protests, however, he is leading the zombie into the bathroom, so John decides not to argue. He is just thinking that maybe they can resolve this situation with minimal drama when there is a knock at the unlocked front door, and Mrs. Hudson pops her head around it.

"Boys?" she says, cheerfully. "What on earth's going on up here? There's a terrible racket. Is it criminals? Do you need me to fetch your gun?"

"No!" yells Sherlock, stabbing the zombie in the shoulder and pushing it through the doorframe, towards the toilet.

"Er," says John, coming over to the landlady. "Nothing to worry about, Mrs. H. Sherlock made a zombie."

"I didn't make a zombie!" shouts Sherlock from the bathroom, although the evidence is still not in his favour.

"Oh," says Mrs. Hudson, which seems an altogether far too reasonable response to this sort of statement. "Well, you need a good head shot then, don't you?"

"We're not going to kill it!" shouts Sherlock. There is a painful-sounding cracking noise, like skull against sink, followed by something disagreeably squelchy that John hopes is just toilet water.

"We are going to kill it," John tells Mrs. Hudson.

"It's probably best," she agrees. "You always hear about the zombie apocalypse, you know, not just the odd one or two. There's no point getting attached to them this early on."

John isn't sure what to say to this, so he just nods. He has always thought Mrs. Hudson's television viewing habits were mostly limited to Richard and Judy's Book Club and Paul O'Grady, so her zombie expertise is a little disarming.

"Can I have a look at it?" she says.

"You probably shouldn't," he says. "Sherlock's just fencing with it at the moment. The idea was to lure it into the bathroom so we could shut it in, but I think he's just doing it for the exercise, now."

There is another timely bang and a triumphant, "a-HA!" from the bathroom.

"Which one was that?" says Mrs. Hudson. "Was that Sherlock or the zombie?"

"He tells me the zombie doesn't talk," says John. "So we can assume that was Sherlock."

"How long have you had this zombie?" she begins to ask, looking curious; but at that point Sherlock dashes out of the bathroom – the zombie has apparently successfully disarmed him, and his foil is nowhere to be seen – slams the door shut and shouts, "We need a barricade!"

There are a few minutes of chaos as Sherlock plasters himself against the banging door and John and Mrs. Hudson scramble to block it off with the sofa and a fully-loaded bookcase, without burying him underneath. One of the long-forgotten mugs of now-cold tea gets kicked over in the process, but no one could care less at this point. Sherlock, when he emerges gloriously from the confusion, is looking rather ruffled; his hair is curling upwards in particularly unruly tufts, and there is a long tear in his dressing gown where he has presumably managed to skewer himself instead of the zombie at some point. On the other hand, he does at least appear to be unharmed.

John means to say, 'thank god you're alright,' but what comes out is, "what an idiot."

Sherlock ignores this.

"I'm angry at you," says John, in case this hasn't been made obvious enough.

"You wanted me to shut it in the bathroom," Sherlock reminds him, breezily. "I shut it in the bathroom. Hello, Mrs. H."

"Hello, Sherlock," she says. "Where did you get the zombie?"

"Borrowed it from the morgue," says Sherlock. John notices that this story has already been minutely twisted enough to make it sound as if Sherlock had been totally aware of what he was doing at the time, and had been doing it under some sort of outside authority.

"He means 'stole'," says John. "He doesn't mean 'borrowed'. And he didn't know it was a zombie."

"John suggested I keep it for the day," Sherlock continues.

This is so dazzlingly unfair that John is briefly incoherent, and then he splutters, "Excuse me, I was trying to make sure you took it back – I'm not the one who went and stole an infected corpse from the morgue just to check its – its decomposition rates or whatever!"

"I wasn't checking its decomposition rates," says Sherlock, sounding insulted. He turns his back on them, swishing his dressing gown again. "I already know decomposition rates."

John tries to share an exasperated look with Mrs. Hudson, but she isn't looking exasperated enough to really build up a sense of fellowship, and there is no point looking exasperated in Sherlock's direction since he is so exasperatingly immune to it. "I'm going to call Mycroft and tell him we have a zombie," he says.

The zombie hammers on the bathroom door. At least, he thinks it is hammering. Maybe it is just running into it, repeatedly. He is not sure how intelligent the zombie is.

"If it breaks that door," Mrs. Hudson tells him, "you two'll have to pay for it, you know."

Sherlock gives a little huff, and tries to peer through the crack to continue his observation.

Mycroft answers the phone with a single word: "Zombie?" He somehow manages to inject these two syllables with more honey than John can keep in the kitchen cupboards, and more poison than Sherlock can keep in the fridge. John is past being surprised at Mycroft's up-to-the-minute knowledge of events on Baker Street, and just hopes that Mycroft only knows about this particular zombie because he has them under surveillance, and not because he was able to deduce it from his ringtone or something equally ridiculous.

"Yes," says John. "Zombie. One of Sherlock's corpses seems to have been infected with something. Now it's trying to eat us."

"Ah," says Mycroft. "Well, you seem to have it under control. Sherlock won't let you shoot it, I suppose?"

"No," says John, and then, because he can't resist the humour of the situation, "Curse his bleeding heart and love for mankind."

"I'll come round with someone to pick it up," says Mycroft. "Tell my little brother to ask if he wants bodies in future. The infected ones were being kept separate."

"I'm not sure you're quite seeing the big picture here," John begins, and then adds, "No, I think you're seeing the big picture, but not the small one, where there's a zombie in our bathroom and that's a problem."

Mycroft has already hung up.

"He's going to come over," John tells Mrs. Hudson.

"I'm going back in there," says Sherlock.

John means to say, 'no, you're not', but instead he says, "Do you have to?"

"I want to study it more," Sherlock announces earnestly. There is an ominous gleam in his pale eyes. "Especially if I can't keep it."

"You can't," say Mrs. Hudson and John, together. John is deeply relieved that this particular statement comes out the way he meant it.

"Fine then," says Sherlock, and shoves the bookcase aside.

XXX

When Mycroft Holmes arrives at 221b Baker Street, twenty minutes later, he cannot help but wonder if 'Zombie' is some sort of code that he's forgotten for 'localised nuclear bomb'. He has a lot of codes to keep track of: it would be easy enough to do. For one thing, someone seems to have upended every bookcase in the flat – and there are a lot – and the sofa is upside down. What was once the bathroom door is now floating untethered in the middle of the living room floor, accompanied by a large chunk of sink. A fencing foil has been plunged in an armchair, up to the hilt. There is a damp patch on the rug where someone has knocked over a cup of tea.

There are four people in the kitchen, but one of them is dead.

One of them is sulking.

Two of them are eating custard creams, which Mycroft eyes up hopefully before he can stop himself.

"May I ask who delivered the final head shot?" he says, crossing one leg over the other and leaning on his umbrella in the doorway.

John looks up. "Mrs. Hudson," he says. "Always prepared for the zombie apocalypse. Who knew?"

Mycroft had known, of course.

But he nods.