The dead dog that was rigged to explode really wasn't the highlight of the day, John muses. He's shucking his absolutely disgusting trousers in the back of an ambulance, trying not to elbow Sherlock, who's stuffed in there with him doing the same thing. He turns around and gets as far as "So, you think the smell will ever – " when he notices the marks.

Long, thin, dark wheals across Sherlock's lower back and upper thighs. He'd lay good money there were some on his buttocks, too, because John may not be a deductive genius, but he is a doctor.

Whip marks.

None so harsh to break the skin, but plenty enough to lay pointillist lines of blood to the surface for a week or more. Sherlock turns an eye over his shoulder as he's taking off his shirt, and catches John staring.

"Problem?"

John swallows. "No, not at all."

…

Sherlock fights to bite back a gasp as the flogger wraps itself around his torso, a full-body thud and a sharp snap as the tails whip around him, and the feeling is so utterly delicious it makes it hard to concentrate.

"Focus, or I'm going to have to gag that pretty mouth."

He nods in supplication, wishing he could say it. "Thank you, Mistress," is on the tip of his tongue. Or would be, if she hadn't forbidden him to speak.

…..

John's tried desperately to forget what he saw nearly a month ago, the evidence of a whip scattered across Sherlock's skin. He hasn't said a word, but it's been nagging at him at every quiet moment. He's never quite understood those kinds of relationships, the power exchange that seems to be inherent in them, and why, for God's sake, Sherlock of all people would surrender that way. Sherlock ceding control to anyone, even for a minute, seems so utterly foreign to the self-determined nature that practically screams itself in every word and deed. He resolves to let it alone – Sherlock's relationships are really none of his business – but when a blue silk sleeve falls away from a pale wrist ringed with red chafe marks, he knows he's got to ask.

"Care to explain that, then?"

Sherlock huffs annoyance. "Why should I? You know perfectly well what they are."

"I – well, a little rough, isn't it? I just wanted-"

"What? To be sure I'm safe?" Sherlock snaps. "I'm perfectly capable of making my own decisions, John, and as you and I aren't shagging, I don't see why you have anything to say about it."

John winces, trying to press on, to satisfy his concern but not really wanting details. "I don't. I really don't want to know, I just want to be sure you're…fine. You know, just in case criminals of the world unite to unleash a paddle on your arse, or something." He shifts uncomfortably, looking down at his plate and really, really wishing the floor would open up and swallow him.

'Hmpf. As if they could. Honestly, John, give me some credit for discretion. We've been living together for almost two years and you didn't even know until that night in Harrow. It's not as if you came home to find me tied to a whipping bench in the sitting room."

John holds up his hands in surrender. "No, no, no details. Stop. I get it. Just – watch yourself, all right? "

Sherlock grins, a predatory gleam of teeth and eyes. "Oh, no worries, John. I know exactly what I'm doing."

…..

"You aren't allowed to punish me for something that happens on the job. We agreed."

Sherlock pants, straining behind the blindfold. He's trying to pinpoint where the voice is coming from, but the sound is echoing around the mostly empty room, making it difficult.

"I'm not. But you better step lightly. You've put me in a mood tonight with that little disappearing act, so no foolishness." The voice, suddenly almost directly in his ear, makes him jump, as does the hand that traces down his spine before grasping his hip. A foot insinuates itself between his and knocks his legs apart, spreading his knees and leaving him open and vulnerable. He shivers, waiting.

"Hands, darling, on the bar. Move them and there'll be hell to pay."

…..

John sometimes hates the internet. It's at once too much information and not enough detail. He's been spending almost all of his time when Sherlock isn't around looking up information about BDSM relationships, and he's somewhere between horrified and fascinated with what he finds. He can barely comprehend the situations he's seen – bondage and riding crops and whatnot not withstanding, the power dynamics of it all, the trust it all requires, just boggles his mind.

His mind, yes, but sometimes…oh, sometimes his body has other ideas. The visceral punch to the gut when he sees a webpage devoted to ropes and knot tying is shocking; he suddenly imagines red ropes against pale skin and Sherlock, on his knees, begging. John slaps the laptop closed, horrified at himself. It's one thing to wonder, another to actively fantasize about one's flatmate and friend, who, moreover, you are decidedly NOT shagging.

He's fine with it. The not-shagging. Sherlock made it clear he has a very specific taste, and he expects to be catered to.

Which John really isn't capable of doing.

Right? Right.

…..

"I…I told you to lie on your back."

Sherlock suppresses a sigh and does his damndest not to roll his eyes. The first time is always the hardest; but once through this hurdle, it should get easier, smoother, better. They almost never revert once they've gotten over the hesitation. Control, and control over him, is so much of a turn on they can't. And then, well, who would deny him anything? He decides to give a little direction, just a hint, and maybe he'll get something out of it tonight himself.

"I'm sorry sir," he says softly, careful to keep it demure. "Would you like to tie my hands, now? I think you'd enjoy it very much."

And so would I, he thinks.

….

"For God's sake, John, stop staring. If you want to know, just ask." Sherlock doesn't even look up from the pipette he's carefully filling from something in an Erlenmeyer flask.

John feels the blush rising up his neck and spreading over his face. He wasn't trying to stare; in fact, he was actively trying NOT to stare, which is where the problem lies. He was uncomfortable in his newfound knowledge, and Sherlock could sense it, like a shark smelling blood in the water.

"What makes you think I have anything to ask?" Stupid, stupid, stupid, do you want him to start talking? He knows that it would only take a few sharply worded questions to unearth everything he ever read, saw, or thought about Sherlock in the last few weeks. He steels himself, ready for the assault.

But it doesn't come. At least, not in the way John was expecting; probing questions and sharp retorts and devastatingly accurate deductions. Instead, Sherlock lays the pipette on the counter and crosses the room to drop gracefully on his knees in front of John's chair, with his hands palms down on his thighs and his head bowed. He looks up at John's shocked face through the dark curls of his fringe.

"I could show you, you know. Answer your questions. I know you've been thinking about it."

John gapes and tries to get a grip on his brain, which seems to have cleaved in two and is at war with itself, between arousal and fear.

"What do you mean? Give a demonstration here in the sitting room?"

"No. I was thinking of something more…practical. I could show you. What I like."

John flushes, heat spreading across his chest. "This is insane," he starts, slightly flustered. "Probably the most insane thing in the litany of insane things you've done. Are you asking me to tie you up and, and what? Beat you? I can't. No." John abruptly stands up, brushing past Sherlock, who is still kneeling, and goes upstairs, slamming his bedroom door in his wake. Christ. Sherlock is really the most impossible man. They're flatmates, not lovers. But leave it to Sherlock to try to solve issue with "practical" demonstration, even if John was interested, which he isn't.

And if he's hard as a steel rod, he's never going to admit it.

…

She does so love role play, Sherlock thinks, straightening his white bow tie in the mirror over the sink. It's the only way she gets to live the life she wishes she could have. And that's fine with him – even with the elaborate set-ups, she's never broken the rules (only he can initiate time, place, and how often, and never in public). And every single time only cements his hold. He grabs the black silk blindfold from the counter and stuffs it in his pocket, checking to make sure that everything else she asked for is somewhere about his person – lube, plug, collar. He pats his jacket, smoothes his lapels, and steps out of the bathroom to a squeal of delight.

"Oh! Gorgeous. So perfect. You're such a good boy."

"Thank you, Mistress."

"Oh, you'll thank me later, no doubt about that. Kiss." She props her foot on the nearby chair, rucking up her already-short skirt. Sherlock goes on one knee in front of her, and presses a reverent kiss to the inside of her stocking-clad thigh.

…

John wakes up sweating and sticky for the fourth night in a row, cursing Sherlock soundly for his predilection for kinky sex and his offer to open a door to that world to John. He's always considered himself a fairly open-minded person when it comes to, well, anything, really, and living with Sherlock for the last 2 years has only reinforced that view. He's had his share of pretty vanilla sex, with men and women, and some a bit more adventurous, but nothing at all like he suspects Sherlock is into. It's certainly something to ponder on, as he's never had an offer quite like it.

Really, the harder question is why Sherlock did offer. It sure as hell wouldn't be out of generosity, so he must want something out of it. Perhaps the idea of a trusted partner – one he knew would keep his confidence and not share. John warms to the idea. Yes, if Sherlock wanted someone to trust, then of course he could trust him. As far as the rest goes, well, Sherlock's a very attractive man; John would have to be dead not to notice. And would it be so hard to order him about a bit? No, he snickers to himself. Maybe I'd order him to do the dishes.

No you wouldn't, the Devil on his shoulder whispers. You know what you'd tell him to do. And he'd do it. John groans and stuffs his pillow over his face. This is getting to be utterly mental. He can't concentrate at work, he can't sleep, and thank God for small mercies that Sherlock's been mostly out for the last week or the sheer volume of laundry and showers he's indulged in would certainly cause comment.

His bedroom door creaks open in the moonlight, and John jerks upright, badly startled. Sherlock is there, standing in the doorway, his eyes barely a gleam in the darkness.

"Sherlock, fuck, you really need to stop creeping around like that."

"Sorry, I heard you moving," he says, and stands there, rather pointedly not leaving.

John laces his hands over his knees and waits. "Something I can help you with?"

"No, but there's something I can help you with," and he crosses the room, climbs onto John's bed, leans over John's bent knees and kisses him.

…

"On your knees. Now."

Sherlock drops to the floor obediently, a thrill of arousal running up his spine at the command. This could be so, so good, he's so much better, stronger than any first time Sherlock's ever experienced. Strong fingers grip his jaw and force his mouth open. He closes his eyes, waiting, shifting his wrists the tiniest fraction where they're bound behind his back, the red ropes causing the slight hint of a burn as he moves. A soft slap to his face pops his eyes open.

"No. Eyes up here, on me. Don't close them, understood?" Sherlock nods, mouth still open. He hears the slide of a zip, and he leans forward to take the cock that's been presented to him into his mouth, never taking his eyes off of his Master's face. He works the head, licking , tasting salty precome and soap, trying to split his focus between giving the best head he's ever managed and watching the expressions (arousal, resolve, affection) flit across the face of the man in front of him. Sherlock takes him deep, sucking him down and using the tiniest hint of teeth that makes him buck and moan. When John gasps and pulls him off by the hair to spend across his lips, Sherlock fights to hold himself in check.

John, still panting, releases him. "I hope you enjoyed that, because that's all you get for a while. Go clean yourself up and wait for me in my room. I'll be in later." When John walks behind him to release the knots holding his wrists behind his back, Sherlock breaks into a feral smile of self-satisfied triumph.

Note:

Every one of Sherlock's conquests is someone you know:

1. Donovan

2. Lestrade

3. Dimmock

4. Molly

5. John