Author's Note: Based on a Soulmates AU prompt found on Tumblr. Thank you for reading!

Part I

Sherlock isn't really thinking about it, the first time it happens. He knows about the phenomenon—that whatever he writes on his skin will appear on his soulmate's, regardless of whether or not he's met her. He knows, but he's never experienced the phenomenon for himself. And he's been writing on his skin for years.

He doesn't expect it when he plucks a marker from his jeans. He doesn't expect it when he presses his back to the grimy wall of the doss house he found near his Uni-assigned dorms. He doesn't expect it when he yanks the cap off with his teeth, shoves the sleeve of his hoodie up on his arm, and writes: I don't want to do this anymore.

He's still young, in his twenties, but he's been doing this for years too. A needle in his vein quiets his mind and gives him the rest he craves with every cell in his body. He doesn't want to do it anymore, but he doesn't know what else to do to make it stop.

He doesn't expect a reply, of course he doesn't, but as he reaches for the needle he notices another messy but distinguishable scroll blooming underneath the words he just wrote on his skin.

She writes in English, but he can tell she's used to writing Cyrillic. He reads: Neither do I.

His mind races with information, data travelling at the speed of light. He knows things, from her messy scroll. He can tell she's left handed from the slant of her letters, and that she doesn't enjoy being exposed from the distance between her words.

Needle forgotten, he lifts his marker and hesitates. And while he waits, something else appears on his skin: I have to go.

And just like that, the decision is made for him. He sags against the wall, drops the marker and reaches for his needle. He doesn't want to do this, but if not this, then what? He sinks the sharp tip into his vein, rests his head against the wall, and lifts his arm to read her words again.

They're already fading.

Part II

The next time it happens, it's only been days.

Sherlock's exiting his dorm, avoiding the crowd of early risers eager to call him a freak at the barest hint of a glance. He can't help that he sees things. He can't help that he knows. He can't help that his mind dashes forth, full-speed, and that if he tries to stop it'll pull him underfoot and tear him apart.

"Oi, where are you going? Come back here and tell us how you did it!" And then, quieter. "Freak."

Sherlock ignores the call and shoves one of his arms into the sleeve of his coat. Mycroft's called and wants to see him. He can't very well show up looking like he's been pricking himself with a needle all week.

He feels the subtle tingle as he snaps the collar, and this time it's on his stomach. His footsteps slow and he lifts the hem of his shirt, brows furrowed as he catches sight of her message.

Did you do it?

She's writing in a hurry, he can tell. And from the location on his body, she's hiding it too. He's not sure what to make of it, but he fumbles for a pen, finds an out of the way bench to sit on, and writes back: Yes. And then he writes again: Did you do it?

Her reply: Yes. And again: Do you regret it?

Sherlock scans his surroundings. A patch of green stretches before him, and people cross it to get to early morning classes. He has a good view, but he's shielded by shrubbery. It feels almost like privacy.

Does he regret it? He doesn't, really—not the way Mycroft would want him to regret it, at any rate.

He writes: No, did you?

I don't know.

Why are you hiding your writing? Who are you hiding it from?

He has to wait a long time for her answer. So long, in fact, he thinks maybe she'd not going to answer at all. And then she does.

She writes: We're not allowed to contact our soulmates. How did you know?

I observe.

It feels surreal—that he has a soulmate, and that she's writing to him. It's not his area. He doesn't think about this because he doesn't want to, but she exists, somewhere, and she is writing to him.

He writes back: What happens if you contact your soulmate?

Punishment. She writes it neat and straightforward, and he can tell she's emotionally detached herself from the concept.

He doesn't know what to write back. He has pieces of a puzzle but he doesn't know how they go together, or what the image is supposed to be. He considers ignoring it, but regardless of what others would be quick to believe, William Sherlock Scott Holmes is not a machine.

Not yet—not where it counts.

She writes back before he can: What was it? The thing you didn't want to do but don't regret?

He hesitates: Morphine. What was yours?

She doesn't write back for a long time again and he assumes he's scared her off. Just as well. He doesn't want a soulmate anyway. He doesn't need one.

He's just about to cap his pen when the skin of his stomach tingles again and he lifts his shirt to see.

I'm an assassin and a spy. And then before he can even process her words: I have to go again. Next time you want to do morphine, write to me.

She adds: On your stomach.

And then again: If you still want to.

Mycroft doesn't ask him why he's late when he reaches his office. He takes one look at him, deduces he's been using, and launches into a thirty minute lecture that leaves Sherlock feeling guilty in his marrow.

He fiddles absently with his shirt in an effort to avoid his brother's gaze, just over his stomach, and remembers the words already fading underneath. He doesn't know how this 'soulmate' phenomenon works. He doesn't know who picks and chooses, and he doesn't like not knowing.

But he does know this: whoever this woman is—and of course he can tell—whatever she does, whatever she says, he is connected to her.

And for the first time since he was little, when he used to cling to Mycroft's hand, Sherlock doesn't feel so alone.

Part III

Natasha writes to him frequently over the years.

She writes to him while she waits for extraction, once she's finished a mission. She writes to him while she takes intermittent peeks through a scope, waiting for her marks. She writes to him when she can't sleep. When there are nightmares. When there's guilt and doubt and fear that makes her chest feel like it's caving in.

He writes to her, too. He writes to her when he's struggling. When he's high. When he's sober. When he makes curious observations or works on his experiments. He tells her about Mycroft. About his parents. About John, when they first meet. She can tell he's cautiously optimistic and she smiles the first time he calls his him friend.

He tells her other things. He tells her about the violin and scribbles his compositions on her skin. She doesn't mention it, but the first time she sees the notes, she sneaks into the ballet studio downstairs, plays the melody until she's gotten the hang of it, and stifles a cry with one of her hands.

She writes to him when S.H.I.E.L.D. comes calling and she decides to leave the KGB. She's not allowed a pen, initially, but when she finds one, she writes the story on her thigh.

She tells him about the agent that came for her. About the deal. She glosses over the details, skips to the parts he'd find interesting or amusing, and he doesn't pressure her to say more than she can handle.

Natasha doesn't know if she's doing the right thing. She doesn't trust S.H.I.E.L.D. to be different from the KGB, because they ask her to do the same things she used to do for the KGB and tell her that now she's working for the good guys.

But she doesn't feel like she's suffocating anymore, and that's something, isn't it? He doesn't question her or judge her and she returns the favor in kind.

If he writes I don't want to do this anymore, she writes I've got you.

If she writes I don't know what I'm doing, he writes I've got you too.

The world isn't kind to them because they are different. Their armor is tough and their weapons are sharp. They are dangerous predators—wolves among sheep—and because the world isn't kind to them, they huddle close, tuck their snouts underneath each other's throat, and they are kind to each other.

Sherlock writes to her after the fall. He tells her about his plan. About Moriarty, and about how he has to leave John behind. He tells her about Molly, about what she'll have to do to help him.

He's hurting, struggling with the decisions he has to make. He's carrying the weight of the world on his shoulders because now it's personal, now the people he cares about are on the line, and when he feels his back about to snap, he writes: I don't want to do this anymore.

Part IV

They've been writing to each other for years by the time they cross paths the very first time.

Natasha's in London, and she's running from S.H.I.E.L.D. She's come full circle. She's leaked her secrets to the internet and presented herself for public scrutiny. Her covers are blown.

She's back to the life she thought she'd left behind. She's been working for the bad guys all along, if such a thing exists anymore. The truth is, she can't tell the difference anymore. She's not sure she ever could.

They've never talked about meeting face to face before.

They know each other's names and they keep track of each other because their faces are now famous, but they know their lives are dangerous. They know that their jobs require sacrifice, that to meet is to expose this miraculous thing they've found in each other, and that is something neither one of them is ready to do.

By now, they know each other too well. They've been writing their secrets on each other's skin for so long that their their veins feel heavy with more ink than blood, and it is a peculiar feeling.

Heavy and pleasant, like they've been claimed.

Once, they played this game—they're both fond of games, particularly when one or both can't sleep—but this game was new, and ongoing since.

Natasha recalls how it started.

Sherlock marked the freckles on his arm, black dots lighting up her skin one by one. Natasha smiled, rolled to her back on the bed, and traced lines between them to make constellations. He then circled one of his scars, and beside it wrote a two-sentence story of how he'd acquired it. He'd fallen from a bicycle when he was little, she still remembers.

And then she'd circled two of her own.

They played their game for hours, until the pleasant tingles lulled them both to sleep. Before she caped her marker and closed her eyes, Natasha wrote one last message for him on her arm, slow and drowsy, but clear: Mine.

It's a strange brand of intimacy, knowing each other's bodies in detail without ever laying eyes on them. It's a strange sort of bond to mark each other with ink, and leave traces deep underneath the skin long after the ink has faded.

Natasha thinks about this when she climbs the stairs to 221B. She thinks about this when walks around his empty living room, and when she studies the papered wall she knows he's christened his 'clue wall'.

She thinks, 'I know this man'. And yet, here she is learning new things.

She sits in his chair, and finds herself enveloped in his scent. She studies his bookshelf, and learns he groups his books not in alphabetical order, but in an echo of the Dewey Decimal.

She breathes in the very atmosphere of his flat, all leather and smoke and ash and chemicals and him, and she learns, suddenly, what she should've learned a long time ago.

Footsteps climb the stairs two at a time. He's alone, and in a hurry to get home, from the sounds of it. He stops short when he reaches the doorway and blinks blue eyes—impossibly blue, unnervingly sharp.

Natasha smiles, tiredly, and feels her body relax for the first time in years. "I don't want to do this anymore," she tells him quietly.

Sherlock shrugs out of his coat and hangs it up beside the door, draping his scarf on top of it. He doesn't say a word, yet, but he strides over and nudges her aside so he can sit. Easy as breathing, like they've been doing this for years, she settles down on his lap.

It doesn't feel strange. It feels natural, like their bodies were made for each other's comfort, and perhaps they were. Is that part of being soulmates? Neither one of them would know.

Sherlock wraps his arms around Natasha, and she tucks her face into the crook of his neck. She breathes him in, and he moves his head just a fraction. His breath against her ear sends shivers down her spine.

And he says, finally, "I've got you."