Disclaimer: I do not own these characters, and am making no profit from their use.
Warnings: Discussion of violence, spoilers for TGG.

One in Ten Thousand

Sherlock had noticed that John had unusual mobility for a shoulder wound, but had never really thought anything of it. Surgery was delicate enough to repair just about anything these days, and John's exercise routine was highly regimented and clearly designed to promote maximum flexibility and strength. It obviously succeeded, as the only time John gave any sign of discomfort was when his shoulder was pulled backwards particularly sharply. Sherlock would have liked to know the precise location of the injury, but refused to look at John's medical records – that would be cheating. Besides, he enjoyed turning the problem over in his mind, musing on the different possibilities.

It wasn't high on the shoulder, that much was clear. If the bullet had hit either the socket or the face of the joint, repair would have been much more difficult and John would have a severely limited range of motion even now. But it couldn't be much lower, or the area wouldn't technically be considered the shoulder any longer – and John was a medical man, after all, and very precise in his terminology. So either the bullet struck the collarbone (which would have healed without complications and could explain John's pain when his arm was pulled backwards) or just below it, where it might have punctured a lung but would have been unlikely to hit anything that couldn't be repaired.

Perhaps 'collarbone or below' was still a little vague, but Sherlock was in no hurry. He liked learning John slowly, unwinding the man's secrets like spools of thread (except they weren't really secrets, not when Sherlock hadn't found anything John wouldn't discuss without a bit of prodding).

Then came the Moriarty, and the pool and the bomb-jacket, and in the moment when he honestly thought he was going to die, the only thing Sherlock regretted was that John would most likely die with him.

But they survived, and staggered back to the flat rather than the police station because after that debacle Sherlock just recoiled at the thought of having to interact with any human being other than John.

"You all right?" John asked, clearly in earnest even though he was the one who'd spent hours at the mercy of a psychopath.

"I wasn't the one kidnapped!" Sherlock snapped, harsher than was strictly necessary.

John nodded with every impression of innocence. "Didn't answer my question."

"I'm fine," Sherlock muttered.

"Good, you can give me a hand, then."

Alarm spiked through Sherlock's brain. "Did he hurt you?"

"He didn't," John said quietly, his eyes fixed on the wallpaper. "Doesn't like to get his hands dirty, remember? But I saw his goons coming, and I didn't make it easy for them."

Of course. John fought, and was subdued with violence and – judging by the use of the plural – superior numbers.

Sherlock's thoughts were momentarily derailed when John – with a slight wince – pulled his jumper off and began unbuttoning his shirt. His back was to Sherlock, but the dim lighting of the sitting room was still bright enough to highlight the dark bruises and bloody scrapes that shadowed his ribs. Some marks had been made by fists, but most by boots – they'd got John on the floor, and they hadn't wanted to risk him getting up again.

For a moment, the fury that rushed through Sherlock's head literally made his hands shake. They'd pushed John to the ground, beat him, kicked him, strapped him to a bomb…

"Nothing broken," John told him, hissing as he probed at his ribs. "Just a bit tender – some antiseptic will do me, there should be some in the cupboard."

It was clearly a hint for Sherlock to get it – to be useful, instead of just standing there picturing inventive deaths for Moriarty – so he pulled the small bottle out of the cupboard and located some of his sterile wipes. Which were supposed to have been used for the saliva experiment, but Sherlock felt no regret at re-purposing them.

John turned, stretching out his hand for the antiseptic…

And Sherlock saw the scar for the first time.

He supposed it was largely ordinary, as scars from bullet wounds went – a small, puckered oval about the size of a fifty-pence piece – but it was the location that had arrested Sherlock's attention.

The bullet wound wasn't in John's shoulder. It was in his chest. Two inches below his collarbone, one half-inch to the left of his sternum. Whoever had shot at John had been aiming to kill, because the little scar was perfectly aligned with John's heart.

"Did it shatter on a rib?"

John looked up, a small line between his eyebrows that meant he was puzzled.

"The bullet," Sherlock clarified, still staring.

John shook his head. "Would have gone clean through, but it ended up chipping off my scapula."

Which made absolutely no sense. For the bullet to have hit John's scapula meant it had to have gone through his chest – through his heart – and Sherlock wasn't a doctor but he was quite confident people didn't survive bullets to the heart, especially if they were in a war zone with no emergency care immediately available.

The first thing he always did when he was presented with a mystery was get closer to it, and that was what he did now. He moved closer to John – closer than most people were strictly comfortable with, but John never seemed to mind – and carefully pressed the tips of two fingers to the scar, prodding it gently.

Sherlock glanced up at the sound of John's chuckle. John didn't look upset or irritated, but only faintly amused and slightly daring, as if he were challenging Sherlock to figure it out.

Sherlock flattened his hand against the scar, feeling the pulse of John's heart beneath his palm.

…thump…thump…thump…

If he pressed his ear to John's chest, he would distinguish two beats – the contraction of the atrium followed by the ventricle – but they came too closely together for his sense of touch to discern the difference.

…thump…thump…thump…

As Sherlock was puzzling over whether he'd ever heard of anyone surviving a bullet to the heart, John chuckled again and took hold of his wrist, lifting it off John's chest. Sherlock was about to protest – how was he supposed to figure it out if John wouldn't let him examine the evidence? – but John simply moved Sherlock's hand to the right side of his sternum, and pressed it hard against the unscarred flesh.

…thump…thump…thump…

This time John's pulse was firmer, harder, each beat of his heart more clearly distinguished. And finally – slow, unforgivably slow – Sherlock understood. He knew this, he'd read of it, and there was a name, what was the name…?

"Situs inversus totalis," John said quietly.

"Extraordinary," Sherlock breathed.

…thump…thump…thump…

The scar wasn't directly over John's heart because John's heart wasn't there. In fact, the position of all his major visceral organs were reversed. How fascinating.

"Can I have the antiseptic now?"

Sherlock handed it over, still drinking in the sight of his hand pressed to the right side of John's chest, feeling John's heartbeat, slow and steady, not picking up in the slightest even as he dabbed the stinging antiseptic over his scrapes.

"You said you were shot in the shoulder," Sherlock couldn't resist pointing out, feeling snippy. How was he supposed to work these things out if John gave him wrong information to start with?

John shrugged. "Reflex. Saying I was shot in the chest tends to require some explanation, but most people don't ask questions if you say you were hit in the shoulder. Personally, I blame all those action movies – nowadays, everyone seems to think you can just walk off a bullet in the shoulder."

…thump…thump…thump…

"Sherlock, I'm fairly certain my heartbeat isn't that fascinating."

Sherlock paid that all the attention it deserved, which was none. "How common is it?"

"Mad geniuses wanting to take my pulse? Or my particular medical condition?"

"Don't be dull."

John sighed, the kind of sigh that said he wanted Sherlock to know just how patient he was being. "It varies across populations, but it's generally held to be about one in ten thousand, maybe a bit less."

…thump…thump…thump…

One in ten thousand. 0.01 percent. A medical anomaly that reversed the positions of John's organs and enabled him to survive a bullet that should have killed him. That let Sherlock meet this man, who thought he was amazing, whose reactions were just like his organs – the mirror-opposite of 'normal'.

John was kidnapped, beaten, and strapped to a bomb because of his association with Sherlock. Anyone else would have gone to the hospital, gone to stay with a friend while they made other living arrangements. John was just looking at him with a faintly concerned frown. It was almost as if he was worried about Sherlock more than himself.

It was completely ridiculous, of course, but some part of him thought it was strangely fitting that John's body should reflect the uniqueness of his personality. Not that one in ten thousand could match John – not even one in ten billion.

…thump, thump, thump…

And now John's heart rate was picking up, anxiety in his eyes. He looked almost as worried as he had back at that horrible pool.

Sherlock wasn't one to dwell on past mistakes besides whatever it took to ensure they never happened again. But it terrified him to think how close they had come to dying. How close John had come to dying. The slightest shift in Moriarty's mercurial whims, one sniper with an itchy trigger finger, and Sherlock would have been standing in the kitchen alone. Without John.

People often said they couldn't imagine what it would be like to lose someone they cared for, but Sherlock could imagine every slightest detail of it, and that was exactly why it terrified him so much.

…thump, thump, thump…

"Hey, don't look like that," John chided, laying his hand over Sherlock's where it rested on his chest.

He curled their fingers together but didn't actually remove Sherlock's palm from his skin, for which Sherlock was ridiculously grateful.

"It'll be alright," John said, his voice low and soothing. "You'll see – everything will be fine."

…thump…thump…thump…

John's heart was slow and steady again, regular as metronome, showing the strength of his conviction. Just for a moment, Sherlock allowed himself to believe.

xx

AN: Thanks so much to ginbitch, my wonderful beta, who fixes up my writing even when she's rushed off her feet.