They tell you in medical school that sooner or later, you will kill someone. Someone will die because of a mistake you made. Because you were tired, you forgot, you misdiagnosed or miscalculated, you were thinking of something else at the time. It happens. You're horrified by the thought, and it's only right that you should be. You believe you will be different, careful. Infallible. Except, of course, you're not. Not always. And you will never forgive yourself for it.

When the Master died, it broke my heart.

I wasn't expecting it to, no. Not that I could have killed him myself. I could give you the whole schtick about how I'm trained to save lives, not destroy them... but it's not that, not really. You see, I think I could have done it at first. Given a gun. Given a gun and my family bound on the other side of the room from me, fearing for their lives and mine. I couldn't have done it in the sight of the Doctor. Not with his eyes on me. But I know the rage I felt. Given a gun, I could have done it, if he wasn't there to hold me back . Not a pleasant fact, but there it is.

But a year... that's a lot of premeditation. That's plenty of time for the blood to run cold. To think, and to plan, and to anticipate. And I'm not that sort of murderer. I don't know whether I'm a murderer at all, but I'm definitely not that sort.

It broke my heart because it broke the Doctor's heart. That's it. Simple as that.

When my mum asks how the Doctor could mourn a man who carried out mass murder on such a scale -- forgive him, weep for him, beg him not to die -- the only answer I have for her is that they were the only two of their kind left. I say to her -- imagine, imagine everyone was gone, and there were no humans left in the entire universe except for you and...

Hitler is the obvious one, of course. You could say Stalin and Mao, for the sake of variety. But all she does is say: they weren't human. Inhuman. It's probably the worst condemnation you can make. Like some crimes are so bad, so irredeemable, that you are exiled from your own species.

There's another explanation I could give, but it's no better. Because he loved him. Then you have to answer how anyone could love a mass murderer, a dictator, a tyrant? How can they love something so evil and still be good? And I can't answer that, so I don't.

Inhuman. It's what the Doctor is.

Let's face it, he wasn't really that great at being human. The essence of him was gone, locked in a fob watch. He was there, but he wasn't. And what I knew, watching him clutch at the Master and plead with him to stay, was that the essence of him was there. That however many stories he told me of diamond skies, I could only scratch the surface of the centuries. I knew what I had been telling people for the last year, borrowing a child's rhetoric to make my case. He burns in the centre of time, he is ancient and forever.

I couldn't stop him burning. I could save the world, but I couldn't save him.

The thing is, I made a mistake. I saw the engraving on the watch-case. I made the genie curious about his own bottle. Before me, he hid himself at the end of the universe, accidentally trying to save humanity. What kept me going, through the burning and the hiding out in hovels and that horrible bout of dysentery -- apart from love, and faith, and hope -- was knowing it was my job to try and put that right.

The people he would have murdered are living their ordinary lives, unaware they've even been spared. But you never can undo things completely, can you? Because the Master is dead, and the Doctor is alone. I could glue myself to his side night and day for another eighteen months, another eighteen years, and he would still be alone because I'm just a human. I was not born under diamond skies and I only have one heart to break.

I would be alone, too. If I stayed with him forever, I would always be alone.

I knew that already. A year is a long time. To think. To plan. To anticipate. I think it was about five months in that I realised: if I got out of this alive, I was getting out.

I was so used to the sound of guns by that time, and the swish of Toclafane blades slicing the air. I was so accustomed to the smell of burning and the lingering orange glow it casts on everything. I was endlessly frustrated by my own hopelessness at getting through to people, when getting through to people was the only part of my mission that really mattered (the British school system isn't so hot on Russian or Mandarin, and thanks to the Master's cannibalisation of the Tardis I couldn't always rely on it to translate for me). Stumbling through encounters with strangers, always wondering whether this one might betray you for a loaf of bread. I was so damn tired. I wanted to go home. And if I couldn't go home, well then I would have to find another home. If my parents and Tish were dead -- no Martha, you can't think that, you can't let yourself think that, you have to be positive, you have to stay strong -- but if, but if, I would find Leo. I would find my friends. I would stare into whatever void was left, and try and build something new, and not run away.

I just wanted to stand still. Catch my breath. Fall asleep in someone else's arms, and be safe. And be loved, really loved, not just tolerated by someone for whom love is just another tool, like the sonic screwdriver or the Tardis; another thing to save the world with, the hook that keeps your safety net in place.

If I stay with him forever, I will always be alone.

So yes, I knew that already. But I didn't feel it until I saw him clutch the Master, until I heard the passion in his voice. However many times I saved him, and however many times the Master tried to destroy him... it wasn't even a competition. I wasn't even in the running. There were centuries of history between them that I couldn't hope to understand.

I reminded myself of that when I crossed the street to the Tardis, afterwards. I thought it would be hard, leaving him. I didn't entirely trust myself to go through with it. But he made it easier. He behaved exactly as I would have expected him to.

And I thought: I cannot do this. Even if I wanted to stick around, even if I could, I cannot deal with business as usual, you leaping around as if nothing had happened, snapping back like a piece of elastic and refusing to grieve because the grief will destroy you. I am not elastic. There are other lives to save, besides yours. I have a life of my own, and I need to save that too.

You know what else I reminded myself of. Tom.

'No' he shouted. He ran outside and they shot him down. I didn't think that could be undone. I couldn't allow myself to think too much about it. I had to focus on thinking the right word at the right time. Doctor. As if I hadn't been lulling myself to sleep with it for months. Dreaming about him, only to wake and find him still not there, always not there. Telling everyone I met that he was ancient and forever, when I had no concrete proof that he was even alive. That wasn't the hard part. People were so desperate for a way out they'd have latched on to anything I told them.

But I was thinking about it now. I was thinking about how he hardly knew me, and knew it was hopeless, but he tried to save me anyway. Because he's a doctor, and that's what we're trained to do, but also because he's human, and we do things that are hopeless, and brave, and serve no purpose.

Also, he was fit. And I was having serious trouble remembering the last time I got laid.

I know that sounds shallow. But... oh, don't believe anyone who tries to tell you unrequited love is romantic. It's the exact opposite. It's the total lack of romance. It's longing and longing, with every single fibre in your body, for that person to touch you because they want to touch you, not because they're dragging you out of danger or performing a genetic transfer or giving you an obligatory hug that's always over too soon. I'd been away from the Doctor a year. Long enough to dull the pain, not long enough to forget what it was like when it was fresh.

The Doctor might be cleverer and braver and more beautiful than any human man I'd ever met, but I was starting to see that didn't rule out the possibility of other men being clever, and brave, and beautiful. And that they might even have some qualities the Doctor didn't. And that even if they weren't better than him, it might not matter. Because they might be better for me.

I don't know if I feel this yet. I know it, certainly. And I'm hopeful that my heart will catch up.

Maybe it's selfish that I couldn't watch him grieve, or refuse to grieve. Maybe it's selfish that I left him alone with his ghosts. But I'm human. Sometimes, I'm going to be selfish. Sometimes I have to be selfish, because otherwise I'm going to lose myself completely.

There are people here who actually need me, and want me, and love me. A family who went through hell because of the choices I made. Friends I haven't talked to for months and months. People I'd like to know better. I met a lot of good, friendly, decent people, when I was saving the world. I'd kind of like to know how some of them are getting on. And I've worked towards becoming a doctor for a very long time, and I'm not going to junk my entire life for anyone in the universe. Not even him. I know that, now. I was away from him for a year, and I survived. Yes, he's alone now. I can't change that, and it breaks my heart. But it doesn't mean I have to be alone too.