If anyone asks, the Doctor and the Master aren't "together". If anyone asks, they're not even friends. Got it? Good. Because there's no way they would like you spreading their secret around. Only a few people know, and those few only know because they weren't important enough to worry about.
What is that? You think I am important. Well, I'm getting to that, so sit down and listen.
For you to really know what the Doctor and the Master have, you have to know their history. They grew up on a planet far away. The system name isn't important, and neither is the planet name. They're long gone, buried in time.
They were boys when they first met. They were both of the same Time Lord house. What is a Time Lord, you ask? Well… they are powerful. Oh yes, they are powerful. They ruled over time when they were here, and now that only two remain, they rule alone.
So, they joined the academy, where they met. Their names were Koschei, that's the Master, and Theta Sigma, that's the Doctor. Though the universe doesn't really know if those are their names. They've been the Doctor and the Master for so long, their old names have died.
They were two rebels of their society. They didn't want to remain on their planet, you see. They wanted to see the universe, to explore. To know what they were sworn to protect. So they planned to escape. They were going to steal a TARDIS and get away.
Only before they could, something terrible happened. You see, there were certain people that didn't like their views, their ideals, and thought that two youngsters such as them should bow quietly and do nothing. They cornered Koschei one day, and were going to hurt him. Koschei called out for Theta, but Theta was nowhere near, so Koschei had to protect himself.
Yes, he did, all right. He fought them off. What? No, that's not the end of the story. There are no happy endings here. For the Time Lords found out what Koschei had done. He had killed one of his attackers, you see. They made him appear before a court.
Well, Theta came and tried to protect his friend. What? Oh, they were friends at this point. They weren't the Doctor and the Master then, my child.
Anyway, Theta couldn't protect Koschei, and Koschei was banished. That night, it is recorded in legend that Theta wept, and Koschei left his name behind.
Theta wasn't allowed to leave his planet, and he remained there until he was very, very old. You see, Time Lords can regenerate. That means that they can make whole new bodies when their old ones are no longer alive. Theta was old by this time, as I said, and he had a family. With daughters and sons, and a single granddaughter. Susan was her name.
But then the people came back. The people who had attacked Koschei attacked Theta. And he killed one of them also. When he appeared in court, it was last time he gave his name as his own. When he was thrown out of Time Lord society, Theta died. Only, many years had passed, and he didn't want to leave his family. So he took his granddaughter with him.
He was now a wanted man. Why? Because he had kidnapped someone, you stupid child. And that is wrong.
When Theta… the Doctor met the Master next, he had changed. The Master blamed the Doctor for his banishment, and tried to hurt him. And so it continued down the centuries. The Master and the Doctor would play their Game, where pain was the prize, but death was never offered.
Only then, a war broke out. The Time Lords had become warriors, reverting to their primitive state. And they had need of the renegades' ideas. They called them back and put them in armour, telling them to protect their golden planet. The two men said yes, because this was their planet and nothing was more important than this.
When the Time Lords fell, it was the Doctor who took the burden. For the Master had run. Yes, child, like a coward. But bravery wasn't something the Doctor possessed either. He hid from what he had done, and when they met again, they found they didn't know each other.
So the Master did what he had always wanted to do: he took over the Earth. Because now there was no Doctor, at least not the one he had known. And he looked over Earth, at the death he had caused, and the Game began again. This time though, the stakes were higher. It wasn't merely one planet; it was their souls on the line.
But the Game had long ago decided to leave the two men to their jesting, and had left a single outcome that would always happen, no matter what the two Time Lords did to change it. The Doctor won, as he always did, took the Master into the TARDIS… and kept him.
So, child, if anyone asks, tell them not the story of the Time Lords. There are songs enough for that, but the asker will have to find them on their own. No, the Doctor and the Master deserve to tell their own story.
So, they are not "together". They are not friends. They are merely the last two of an extinct race, trapped in an endless dance of futility, which will always end the same, because the Game got tired, and left.