Sherlock’s heart, by the time he is thirty, is a mess - haphazard stitches and overlaps, torn threads and thinning muscle - barely beating in his chest. Good thing, then, that it’s only the seat of all feelings, and not something he needs in order to live.
Mycroft’s is neater - only a few professional stitches from a time in his teens. His heart beats irregularly, but is in far better condition. He reminds Sherlock at every opportunity - caring is not an advantage.
Sherlock tries not to care, he becomes a self-titled sociopath, but the truth is his heart still beats with a tiny hope that someone will understand him and perhaps care. Hope, he thinks, is what wounds him, but it’s so hard to shed.
From time to time he opens up his chest to search for the exact spot hope springs from, but over time Sherlock thinks it must just be a poison in his blood that makes his nearly-unfeeling heart beat just so, like a dying bird with a last flutter of wings.
When he meets John Watson, Sherlock has to concentrate to breathe. His heart suddenly picks up speed because here is someone new, here is a possibility to be understood, and his stitches hurt because they’re pulling at the seams as his hope and excitement enlarge his heart. All he can think is not again because hope always does this, and it’s always wrong. Still, he breathes and deduces, ignoring the pain of his old stitching as hope swells.
He deduces: a doctor, an army doctor, a stoic possibly, and a realist surely. An utterly normal man, except for his psychosomatic limp and his injured shoulder, but just possibly a person who could tolerate Sherlock’s foibles. No, he won’t consider it. Stamford may have brought him, but that doesn’t mean anything - he tries to tell his heart that, but it’s pounding with anticipation. He shouldn’t even consider the possibility.
But then John Watson says “Brilliant” and there’s a twist in his chest as he smiles at John, surprise on his face.
If every day can be like this, he can tolerate the hope/joy/pain of his heart in his chest.
Months later they’re panting in an alleyway. Sherlock has a slice across his throat that’s dripping blood, and the thug is out cold on the pavement thanks to John’s quick thinking and hard fist.
“Are you alright?” John asks, approaching, maneuvering his head to get a better look at the cut, but Sherlock can’t hear him over his galloping heart - only knows what’s said because he can read lips.
So Sherlock stands, trembling with adrenaline, while John reassures himself that the cut isn’t dangerous. Lestrade and the others appear, running to take custody of the criminal lying prone, and the lights from their cars and the ambulance play across the brickwork.
“It’s fine, John. Stop prodding.”
“Right. Looks fine. Needs a bandage. Just...” John sighs, licks his lips and steps back to stare up at Sherlock. “Stop being such an idiot when someone has a knife to your throat.”
His heart twists at John’s expression - concern, frustration, and ...affection - and he muffles a groan at the sensation in his chest.
“Nothing, John. I’m fine. As you can see.” He brushes gloved fingers just below his cut. “Lestrade is waiting.”
“Right.” Frowning, John trails behind.
As Sherlock hails the Inspector he thinks, of course John cares. He’s seen it, observed its growth in all the small things John does. But he thinks that if he could just open up John’s chest, then he’d know for certain exactly what he feels. But hearts are a private thing - John would probably say that was a bit not good.
Each person guards their heart jealously, keeping their chests carefully closed until alone, unless their company happens to be trusted - a lover or a doctor there to make sure the heart doesn’t completely fail.
Sherlock has never opened his chest for anyone, but that doesn’t matter - he doesn’t have to show his heart for it to be hurt.
Still, Sherlock waits until John has retired for the night before he opens himself up.
He’s done checks since John moved in - quick, between cases - watching his heart turn vibrant with affection he still tries to deny, enlarged just slightly with pleasure and hope. The truth is, it hurts - his improper repairs make the experience painful. Since John moved in, it’s begun healing itself, and now it’s trying to heal around threads he imbued while tired and broken, fearful or drugged. Sherlock knows he should fix them, cut them out, but caring is a disadvantage and he can’t help but think that John will eventually leave. So each time he inspects, he simply closes himself up.
If nothing else, he knows what a healing heart looks like, the stages it takes. He sees the flesh knitting together, the muscle strengthening as his heart beats more steadily than it has in years. He takes careful notes - he’s his own experiment.
Weeks have gone by and it’s long past midnight when Sherlock slumps back on the sofa in the dimly lit living room, listening as John flips off the bathroom light and climbs the stairs to his room. Once he hears the tell-tale rustling of John readying himself for bed, he opens up his chest to relieve the pressure of the terror and concern that’s made his heart twist and slam and quake. They survived the confrontation at the bridge, they’re safe, but John almost died tonight.
There had been only a pair of the gang members left, but Sherlock failed to observe the gun in one’s pocket - he watched in slow-motion, his heart straining under the weight of horror, leaping in desperation as John was hit before Sherlock brought the criminal down. Luckily, just a graze, but what if it hadn’t been? His mind, usually so concerned with facts, swirls with what-ifs.
He rests his head against the back of the sofa, closes his eyes, and tries to calm his heart. He thinks of other moments - John fixing tea, his laugh, the way he licks his lip when he’s thinking. Sherlock feels his heart swell, and groans softly because it aches with old pains and new love. He knows it’s love now - it must be. He hasn’t felt so strongly before - it feels like it’s consuming him.
He’s so lost in these thoughts, that he doesn’t register the touch on his shoulder. John’s worried “Sherlock?” pierces through his thoughts and he opens his eyes to meet John’s troubled gaze.
Ah, his chest is still open, and fear makes his heart shrink back. He casts his eye to the bandage peeking out from John’s t-shirt sleeve, neatly avoiding his gaze.
“It’s fine, John.” He heads off the inevitable questions without bothering to close himself up. “I repaired it myself.”
"Sherlock, you...” John purses his lips and straightens as he stares at Sherlock’s heart. “Right. Stay there.” And he strides out of the living room, feet thunking on the stairs, leaving Sherlock staring after him.
He comes back with his medical kit, setting it in the floor and taking a seat next to him. He pulls out scissors, needle and thread, and rearranges Sherlock to his liking.
“What are you - “
“Your repairs are bloody awful. Threads imbued with desperation? Christ. What’d you research before you made them? A bloody romance novel?”
“John, I’ll have you know - “
“Shut up, sit still.”
Sherlock’s mouth closes with a click, and he taps his fingers for a few moments before his attention is riveted by John’s movements. He snips out the old bad stitches - hopeless, sorrowful, drugged. His hands are sure - no trace of tremor, and his face is a mask of concentration. He leaves stitches off where Sherlock’s heart is healed enough to do without, replaces others with red threads of care, and, Sherlock’s heart trembles as it beats, with love.
“Oh.” He says, hands relaxing, head tilting back as John frees his heart from his own ill-thought repairs. Sherlock hasn’t felt like this since he was a child and his mother stitched up the little childhood pains, and even then, not like this.
A disbelieving chuckle bubbles up as John snips the end of the last stitch and closes him up with gentle hands.
“Hm? Something funny?” John asks as he puts his kit up.
“You love me.” Sherlock raises his head, and the bemused expression is one John hasn’t seen before.
“For someone as brilliant as you are, I would’ve thought you’d figure that out sooner.” He closes his case and smiles.
You should sleep.” He stands, kisses Sherlock on the forehead and steps around the coffee table, heading towards the stairs.
“Hm?” He turns.
“I love you, too.”
John's lips curl into a broad grin.