Dear Doctor,

I don't really believe you will ever see this, but I can't help hoping. So many times we have cheated death, you and I. Yet I saw the footage from the dalek ship. I watched the precise moment when I activated the teleport, switching myself into the safety of your TARDIS and you into the path of a thousand dalek lasers. I know you never survived. I watched your extermination.

Meanwhile I, in your TARDIS in the town of Christmas, found myself in a most awkward predicament. Your companion was completely flabbergasted. And using my carefully cultivated traits of self-preservation and deception, I pretended to be you. I didn't know how much you'd told Clara about regeneration, but I hoped to convince her that I, in my gray-haired and wrinkled form, was the Doctor. I shouted some nonsense about kidneys that I thought sounded worthy of your arbitrary rantings, and then asked her if she knew how to fly your TARDIS, all the while playing the part, almost subconsciously, of a newly-regenerated Doctor.

Things only went downhill from there. What was I to do? Your companion seemed instinctively against this new version of you-an intuition she perhaps should have trusted. I could have dropped her off on any planet, stolen your TARDIS, and moved on about my business. Only, you see, I couldn't.

After all the armies and individuals who have tried to take your life, it was I who accomplished it in the end. It was I who killed you. I, the Master, your oldest friend, was the one who caused the end of the Doctor. I murdered you, not in some grand explosion or intricate scheme, but in a simple last-ditch teleport to save my own life.

I was a villain of villains, and finally I realized it. After so many centuries I looked back on my life and realized the true horror of what I had become, living a life so filled with lies and betrayal that had escalated worse and worse until a final culmination in which I murdered my best friend. You were possibly my only friend, Doctor, and of my own evil designs that was so. Any support or show of friendship I returned with cruelty and betrayal. Only you kept trusting, kept hoping, that someday I might actually change.

All these thoughts raced through my head with unbearable realization as I flew in your TARDIS with your friend, Clara. And, as your companion watched with shock who she believed to be the Doctor, I knew with a terrible, sinking certainty that I had deprived the universe of a hero. Billions would mourn your death, and how many millions more would see their chance to wreak havoc on worlds and commit bloody genocide that they were terrified to do while you, the Doctor, still lived?

And in that moment, I knew what I must do. I can never repay the universe for the horrific deeds I have done, for the unthinkable multitudes I have killed or hurt in some way. These things will forever haunt me-I know that. But perhaps in some small way I could help the universe, give it hope, by dedicating my life to keeping the Doctor alive, and I determined that fateful night to do just that.

But men are not easily changed, Doctor. I suspect you knew that, although you kept hoping for me. I began adventuring, going from here to there in your TARDIS. I wanted to be good-indeed I determined to be good, like you, Doctor. But willpower is not everything. I always fancied I could be good if I ever grew insane enough to try it, but now that I actually do attempt I find it seemingly impossible. "Am I a good man?" I asked Clara. She didn't know the answer.

No matter how much I try to act like you, to be the saver of people and the healer of worlds, I fail miserably. And, in a still worse turn of events, I fear that I am turning Clara more like me. I hoped she would help me, that by her expectation of the Doctor I would become the Doctor. But instead she has started lying and deceiving even more, becoming what I now see as a chilling reflection of myself.

I have saved the people of Earth now. It's fitting that the first planet I ever saved was your favorite, the world of the primitive humans. I investigated a good dalek to see if such a contradiction was even possible, and the creature saw my own darkness hidden deep inside, a reminder that I was not you-could I ever be like you?

I took Clara to meet Robin Hood-the man does actually exist! I dueled him with a spoon. You would have been proud, Doctor. At the end of our adventure I admitted to him that I wasn't a hero. He said "Well, neither am I. But if we both keep pretending to be, perhaps others will be heroes in our name. Perhaps we will both be stories. And may those stories never end." That's why I'm fighting, Doctor. That's why I'm trying my best to be a hero in your name-so that your story will never end.

I acted like a caretaker at Clara's school. It took me back to my old days of disguises and infiltration. Your Clara is a bright one though, and recognized me immediately. I learned that the Moon is an egg-did you ever see that one coming, Doctor? I robbed a bank, though not for the selfish reasons I would have a decade ago.

I defeated a mummy, saving someone else's life and in doing so coming seconds from losing my own. I never thought it could feel so good to give my life for someone else. Is this how you felt, Doctor, when you risked your life for others time and time again?

But my centuries of habit are not dying easily, Doctor. I constantly find myself falling into lies and deception, my constant companions for hundreds of years and now my recurring nightmares. My darkness has begun to rub off on Clara. P.E.-or Danny-tried to help her, tried to get her away from me, but she stayed. I craved her approval, for her to say that I was really the Doctor she knew, but instead I pulled her closer toward the Master, that despicable creature I used to be. Time and again she stuck by my side, and I watch in horror as I now see my own dark glow in her eyes.

Only when Danny died did she betray me. I foiled her attempt and was in no real danger at all as I watched to see how far she would go. She betrayed me utterly, and I ate up the feeling. Betrayal was something I gave bountifully to you, and I cherished the hollow pain with a sort of joy at the feeling, caring Timelord I had become. Could I cast her aside after that? After all I had done to her, and when at last she betrayed me? I could not. I thought of the way you never lost hope for me, never cast aside our friendship, and I told her I could never stop caring for her just because she betrayed me.

A short time ago I met a woman named Missy, a soldier I recognized from a brief visit to Messaline. In a new way I really did feel like the Doctor, because not only was Missy completely insane, but she claimed she was me-the Master. A battle of wits, the Doctor versus the Master, only this time there was a bizarre twist. I claimed to be you and Missy claimed to be me.

P.E., the soldier, was turned into a cyberman, but he somehow managed to keep his emotions. He wanted them deleted. If I did as he asked I would gain a tactical advantage, but if I denied his request in order to preserve his humanity-what was left of it by that point-I would sacrifice the whole human race. What would you have done, Doctor? Did you face dilemmas like this? It is so hard to be good, but it is so rewarding.

In another twist, Missy gave me control of the entire cybermen army she'd created from Earth's dead. This was the true test, a final battle against myself by myself, a choice at a crossroads. I had been the Master, and I had been the Doctor. I am a rubbish Doctor, and perhaps someday if we ever meet again you will see with horror what I did to your name. But I could not go back to being the Master. I never could. My sole prayer was keeping your story alive, and so I threw the control to P.E., and he gave his life-if you can call it that-to save Earth.

Clara almost killed Missy, but I determined to do it for her. I would rather take blood on my hands for one more life than to let the Doctor's companion get blood on hers. But in the end it was your old friend, the Brigadier, who killed Missy; a cyberman from her own army was her final demise. I've wondered every day since whether Lethbridge-Stewart would have done it if he'd known who I really was.

Sometimes I don't think I can live with myself. I killed you, Doctor, murdered you in cold blood as surely as if I'd pulled the trigger myself. I have tried to be you, I have tried so hard, but really I am a horrid Doctor, and I fear I have tainted your name irreparably.

But I shall continue to be the Doctor. The Master is dead and the Doctor lives. The saver of worlds and the destroyer of monsters, the name of the Doctor shall live until I myself die, and I will try my hardest to be worthy of your name.

And perhaps someday, if by some miracle you did survive, we will find each other and you can answer my question. I will live every day in anticipation of that moment, and do my best to act such that your answer will be favorable. I may never meet you again this side of life, Doctor, and I do not know if there is a life after this. But if there's one thing you taught me it's hope. And I hold onto that faint glimmer, that thin strand of hoping that somehow you did survive, and that someday we will meet and I will ask you.

Doctor, am I a good man?