Author's Notes:
The occasional changes of gendered pronouns in this fic may be more confusing than I think they are, but there's really no way round that if you're writing about these characters over a long enough period of their lives. It's just the canon TV pairings, shouldn't be too hard too keep track of. I hope.

Title is a lyric from Love Will Tear Us Apart Again because... well.


The marks on their skin have been there since they were children, when one of them was left in a dusty orphanage and the other was housed on a sprawling family estate. Whatever makes such marks is clever and culturally-adaptive, writing neat and perfectly legible Circular Gallifreyan on their skin long before either of them has the capacity to read it.

The Doctor (who isn’t the Doctor yet and won’t be for quite some time) gets one of the older boys to sound out the name so that he can hear it (so that he can start listening out for it), and he repeats it a few times aloud as if it didn’t sink itself into his memory the first time, as if he could easily forget the name of his soulmate if he doesn’t keep it fresh on his tongue.

It’s an unremarkable and fairly common Gallifreyan name, but to the boy who isn’t the Doctor yet it’s a beautiful string of sounds that proves he has a future.

Someone tells him that his soulmate’s name is ugly, trying to provoke him. The insult leads to a fight, which the Doctor loses almost as soon as it starts. He gets back to his feet having learned a lesson – losing a fight is both painful and embarrassing.

Most children would (quite sensibly, correctly) decide to avoid getting into fights from now on. The Doctor (absurdly, stupidly) decides never to lose from now on.

It’s an insult to the very nature of reality that this turns out to be more or less the right decision.


-


When the Doctor is eight years old he is sent to school despite the obvious-to-everyone-else fact that he doesn’t belong there.

He arrives with dust in his robes, which barely fit him anyway because they used to belong to someone else, and he drops his bag on the floor of his new room rather carelessly.

Less than a minute later he and his new room-mate have learned each other’s names and everything – absolutely everything – has changed, forever.


-


They make a pact to visit every star that they can see in the night sky – together, of course, because why would they ever choose to part? The two of them are soulmates, after all, destinies twisted around each other, literally inseparable on a deep, innate (and rather poorly-understood) cosmic level.

The Doctor (who still isn’t) picks a star at random and points up at it. “That one first,” he announces.

The Master (who also isn’t) shakes his head and points to a different star on the other of the sky. “That one,” he counters.

They glare at each other for a moment and then they begin to laugh (of course together, at exactly the same time). It doesn’t matter if they disagree on the details, because their names are sunk into each other’s skin, bold and unmistakable, and everyone knows what that means.

(It doesn’t occur to either of them that every rule has an exception, even though both of them tend to thrive on that very fact.)


-


There is no single event that breaks them apart, just a series of increasingly vehement disagreements on issues both minor and of world-shattering importance. (Not literally world-shattering. Not yet, anyway. That comes much, much later.)

They do have what seems at the time like a final argument, where they throw everything they can think of at each other, lashing out because it hurts too much not to.

Each of them, asked separately, would claim to be the one to have drawn the final line under everything.

“Get out of my life!”

“Get out of mine!”

Believing that this might actually be possible, the Doctor does his very best to do just that.

(A careful biographer might note this as the last time the Doctor ever allowed himself to be naive. This is just one of the many reasons that the Doctor makes sure that nobody ever knows enough to be able to write the story of his life.)


-


They do actually spend an impressive amount of time apart before the Master shows up again while the Doctor is exiled to Earth. (It’s very important to the Doctor that he didn’t go looking for him. It’s important to the Master too. It doesn’t matter in the slightest to anyone else.)

He doesn’t tell any of his new friends just what the Master is to him – people are shocked enough by the idea that they used to be friends, there’s no point admitting to any of them that it’s far, far worse than that.

They fight each other and they fight together and then they fight each other again. They establish a new pattern for their relationship – trying to kill each other, instantly regretting it, and then pretending that they had no other choice.

Despite all these attempts at mutual murder neither of them will ever let the other die. That would be far too straightforward (and far too kind for at least one of them to even contemplate).

The Doctor considers that at least if this were something as mundane as a marriage it could be ended with a bit of paperwork – ‘irreconcilable differences’ would be a good enough excuse.

(‘Irreconcilable similarities’ might be more accurate, but neither of them is ever going to admit that.)


-


Hundreds of years later, the Doctor (who once again is not called the Doctor – resolutely not) presses a button and kills everyone that they grew up with and all the rest as well.

Afterwards he checks his skin, reluctantly, to see if this has changed his supposed destiny.

It hasn’t.

He accepts this as a welcome punishment, a fitting punchline to a terrible joke.

And then he walks away from winning and losing the Time War and becomes the Doctor again.


-


It’s only when he finds out that the Master isn’t actually dead after all that he realises he never really believed it. If the Doctor is alive then the Master must be alive, and vice versa.

On the Valiant they are together again for an entire year, a year in which the Doctor is tormented and abused and threatened and – the part which renders the all rest of it trivial – not lonely.

While Martha Jones walks the world below to save it from the Master, the Doctor bides his time and basks (guilty but nowhere near guilty enough) in the burning glow, the seemingly unquenchable fire, of his soulmate.

At the end of that year the Master dies (deliberately) in the Doctor’s arms, and even as he screams out his sorrow at the loss the Doctor still doesn’t quite believe it.

(It will later transpire that he was right to have these doubts – the Master isn’t letting him get out of whatever the two of them are trapped in together that easily.)


-


There are only a handful of people left who can understand Circular Gallifreyan, the forgotten script of a forgotten world. A few linguists, a couple of historians, one or two morbidly-fascinated nerds. (The Doctor himself doesn’t count, of course.) It’s a vanishingly small number of people. Hardly anyone at all, really.

Unfortunately, River Song is one of them.

The Doctor hesitates, prevaricates, and delays as he tries to work out how to tell his wife that she isn’t his soulmate before his skin betrays him on its own.

It does, in the end, because while he is very good at talking he is also terrible at talking. He waits, mouth dry and hearts racing, for her reaction when she finally sees the truth. She’ll be sad. She’ll be angry. She’ll be disgusted.

River reads the spiralling name on the Doctor’s skin and blinks (once, twice), before lifting a hand to caress his cheek, and when she speaks her voice is so soft he worries it might suffocate him.

“I’m sorry,” she says, so painfully gently. “I’m so, so sorry.”

The Doctor finds his voice again and tells her, “So am I.”


-


“Maybe this is why we still have those lovely matching tattoos,” suggests Missy, about a week into her stay in the Vault.

This possibility has occurred to the Doctor too (of course it has). “I doubt it,” he says. As a general rule he always does his best to keep hope alive in any situation, but with the Master he takes the opposite approach – it hurts too much when she inevitably stabs him in the back no matter how hard he’s tried to make things better this time round.

“We’ve got a thousand years to get through,” she says. “That’s a very long time, you know.”

“Oh, I know.”

Missy smiles. “I’m looking forward to it, I really am. I give it about a century before one of us kills the other just to get it all over and done with.”

“Well, given your general inability to actually stay dead -”

“Oh, shush! You love that just as much as I do, don’t deny it! Death is for other people. You and I make sure of that, don’t we, with all the killing we do?”

“I don’t -”

“Don’t be modest, Doctor, it doesn’t suit you at all.”

He leaves the Vault without another word, and sends Nardole in his place whenever she needs things from the outside world.

The Doctor stays away for almost an entire week, an achievement he has every right to be proud of, given the circumstances and their history and their… everything.

Missy makes fun of him for it when he returns to the Vault, but it’s no more than he expected or deserves.


-


Gallifrey is gone (again) and she still has his name written on her skin.

She takes a shower to wash away those last lingering hopes and then she covers the undamaged skin with a bandage, because the last thing she wants to see right now is that name, taunting her with its continued refusal to disappear or even fade a little.

“I always used to hope it’d vanish when I regenerated,” he tells her later, when they meet on Earth yet again.

“Not any more?” she asks, curious as ever (and just as foolish too).

He shrugs. “These days I just sort of hope I won’t regenerate at all,” he says, like that’s nothing, like it doesn’t matter.

But the thought of him gone, forever, for good (or bad), for keeps, again, takes her breath away as it always does.

“Do you know how much it hurts when I think you’re dead?” she asks.

“Probably about as much as it hurts when I think you’re dead,” he answers, unimpressed.

“Stop trying to kill me then,” she suggests.

He laughs at that. “Never,” he says, with a wink and an oddly suggestive smirk, and the Doctor tells herself for the millionth time that she hates him. She does hate him. They hate each other. They are the worst possible pairing of souls, an absolute joke of a couple.

And yet she still has his name written on her ever-renewing skin – just as he has hers on his – and always will.

The two of them are soulmates, after all, and just as soulmates should be it seems that they are going to be together forever.

It’s terribly romantic.

(Dreadfully, appallingly, abysmally.)