When they are ten years old, Koschei and Theta run away together.
Each of them is convinced it's his own idea. They both hate the stultifying boredom of their lessons, and they both want a chance to be free; it's one reason why they're such good friends. But with any two friends, one always has to be braver, more independent. One has to be the leader, which logically makes the other his companion and follower. Theta and Koschei have absolutely no doubt as to which of them is which, although they would never agree on the answer.
The truth is that they talk each other into it, each enthusiastically seconding the other's desire for escape while secretly entertaining thoughts of adventure, a chance to prove himself, or the illicit thrill of defying the adults' rules.
They live for eight Gallifreyan days outside the Dome, all by themselves.
Immediately, they do all the forbidden things. They teach themselves to swim in the river, daring each other to stay under longer and longer, wondering what it would be like to drown and regenerate in the wilderness, together. They sneak up the wall of the Citadel Dome at night and write the looping symbols of their names on the exterior in mud. They gaze at the stars and plan travel itineraries for when they're older, choosing the places where they'll go and interfere and dare the Time Lords to stop them.
They chase wild animals, although the only time Koschei catches one, Theta begs him to let it go. Koschei does so, because it's more fun to mock Theta's tender-heartedness after indulging it than to deal with the unpleasantness of his crying. Besides, although Koschei finds the idea of killing the creature intriguing, he feels only disgust at the thought of blood spurting out to stain his clothes, and at the primitive, barbaric notion of rending the animal's flesh with his teeth.
They take to stealing food from wandering groups of Outsiders instead. At least, they do until Theta decides he'd rather be their friend, and Koschei responds by kicking their food supplies into the fire and stomping off.
It takes Theta half a day to find him, and although Koschei plays at being angry, something warm is swelling deep in his chest when the other boy runs to him, scolding and hugging and scolding again. Theta doesn't exactly apologize for consorting with adults after going to all that trouble to get away from them, but after that it's just the two of them until they get bored and decide to go home. So that's perfectly all right.
It won't occur to them until many years later that they were never truly unobserved, that both the freedom and the danger of their experience were largely illusions. When it does, Theta will laugh, Koschei will seethe with anger, and neither will quite understand how similar the two reactions are.
2. Going Places
When they are seventy, full in the flush of Time Lord adolescence, they "borrow" a TARDIS, trick a professor into signing some authorization forms, and go on a holiday together.
They don't dare interfere very much, or it will surely ruin whatever chance they might have of being trusted with a TARDIS of their own, but they have a finely developed sense of just how far rules can be bent before anything finally breaks. A quick trip to observe a primitive species in its native habitat does no harm. Neither does a night spent sampling the fine wines and luxuries of a decadent, sophisticated empire, although the friendly argument over which to do first might result in an added scuff mark or two on the secondhand TARDIS's already battered console.
When they tire of such pursuits, they decide to go stargazing. They observe the first ignition of a newborn sun, then pop forward a few million years to witness its supernova death throes. They dart in and out of wispy, colorful nebulae, idly chasing the spaceborne creatures that call the dust clouds home. They stare into the heart of a spacetime rift. They float in the immense darkness of the intergalactic void, looking out at the pinprick lights of distant galaxies, and they talk about what they'll do out there in the universe one day: practical jokes, feats of daring, discoveries that will have Gallifrey talking for centuries.
The young man who is now beginning to tire of being called "Theta" never quite notices that when they speak of such things, they almost always use the word "we." The young man who has started to think that he should perhaps have another name than "Koschei" does, though he scarcely regards it as a thing worth pointing out. Neither of them thinks of this trip, or of their plans together, as romantic. Possibly this is only because neither of them has ever been taught the word. In Gallifreyan, it is an obscure, anachronistic term.
Still, when they return to face the official disapproval awaiting them, Koschei's arm creeps possessively around Theta's shoulders, and Theta cannot stop himself from grinning.
3. Life Happens
They graduate. They talk -- one of them wistful, the other angry -- of stealing a TARDIS and going off together, although they no longer have any idea where they might go.
But there are rules that even they are not prepared to break, at least not yet. There are family obligations. There is, perhaps, a sneaking suspicion that if they did attempt to spend their lives together, their increasingly frequent arguments might prove fatal to themselves, or their relationship, or both. In the end, it is decided by mutual agreement that not only is there still too much to be done on Gallifrey and that they are far too young to burn their metaphorical bridges, but also that the Master is overbearing and the Doctor frivolous, that they're both far too arrogant to be borne, that each thinks the other is a terrible kisser, and that they are far better off without each other.
Which of them comes closest to believing this is an open question.
When they are much, much older, although still painfully young by their people's standards, the Master invites the Doctor to the home that he has made. He shows it off proudly: the fine, tastefully decorated palace he has had constructed, the planet full of people who look up to him as their ruler and protector, the powerful technology he has created, which will put the universe at their feet and keep even the Time Lords from interfering with them if they do not wish it.
"Stay with me," he says. A proposal or a command, he sees no reason to draw distinctions between the two.
The Doctor looks first at the expression of desperate forgiveness in the Master's eyes, then at the smirk on his mouth, then the cowering servants and the opulent, slave-built palace around them. "My dear fellow," he says, "have you completely lost your mind?"
Wresting away the knife the Master attempts to throw at him, the Doctor shakes his head in disgust. He goes back to Gallifrey, to the woman his family have chosen for him, and begins teaching himself not to think too much about the past.
The Master burns down the palace, and half the planet with it.
5. Both Time Lords
Lifetimes later at least they're on speaking terms again, even if most of the terms are insults. The Master finds he's quite enjoying this phase of their relationship; being the Doctor's enemy may well be the only thing in the universe more entertainingly challenging than being his friend.
Of course, there is the intriguing possibility that the two might not be mutually exclusive. It's an idea he willingly, if briefly, entertains when the Doctor proposes that they run away together and leave the Earth, that unworthy target of the Doctor's incomprehensible affection, to end its days as a buffet table for the Axons.
In his hearts, he knows it's almost certainly a trick. The Doctor has always been the type to declare enduring loyalty, then leave without a backward glance. And yet he cannot help but feel a tingle of excitement as he considers it: the two of them travelling time and space together, each pretending to cool aloofness as they circle each other, looking for signs of something much less detached and Gallifreyan beneath... It's a dance they have done before, and the Master cannot deny that he has found the results satisfying in the past. The thought is made even sweeter by the fact that the Doctor's knowledge of time travel has been stripped from him, giving the Master the advantage and the Doctor the position of humble dependency. As it should be.
When the inevitable betrayal comes, he feels surprisingly little anger, and the annoyance he experiences is mingled with a strange sense of warmth. Such manipulative tactics remind the Master pleasantly of himself, reconfirming a truth that he has always known. No matter what sanctimonious lies the Doctor might choose to believe, they are much more alike than they are different.
Isn't that what always keeps him coming back?
6. World Without End
The next time the Doctor makes the offer, he is sincere, and the Master's rejection appears to be final.
But the time after that...
The Doctor sits, strapped to the chair, and softly makes his plea. When he finishes, there's an expression on the Master's face he hasn't seen since they were children. Uncertainty. Uncertainty and vulnerability. The Master, he understands, is as alone and adrift as he is and has been for a very, very long time. Donna was righter than she knew; renegades like them, they need someone to help keep them sane.
He wants to reach out for the Master, but the restraints won't allow it. So he sits there, saying nothing, and waits for the universe to change.
The moment passes. Or, perhaps more accurately, is interrupted. Placed in suspended animation indefinitely.
But it's all right. The Doctor knows there will be another chance, even if one of them is dying and the other is trapped in a conflict somewhere outside space and time.
Somehow, there always is.