If there was one thing the Doctor feared, it was being alone, and by alone, it meant in the complete sense of the word.
To be totally and utterly alone.
Sure, there was the Master so he wouldn't be completely alone, but every now and again he would wander off, not to be seen again anywhere from a few days to a few months later, when he would suddenly turn up out of nowhere, be it in the middle of a crisis with the Doctor needing someone's, anyone's help. Or it would be one of the bad times where he locked himself in the TARDIS on the last planet or space station or wherever it was that they were last together and just let himself remember.
And he remembered everything.
All the times he had been with the Master, all the times they had laughed or cried or been imprisoned (and that had been a lot of times). All the times they had shared secret smiles or secret glances or the first time they had kissed when he had brought the Master with him after the year that never was. Or the first time they had made love. And all the times afterwards and all the times the Master had just held and comforted him as he sat there, numb after a particularly bad adventure where many, too many, had died.
For him. For their country, their planet, their home. Or they were innocent and were in the wrong place at the wrong time and no matter what, the Doctor just. Couldn't. Save them.
And they just sat there, together, wherever it was that the Doctor decided to finally just drop his façade of 'Everything's fine, I'm okay!' and grieve. The Master sitting there with him, holding him in his arms, just holding him. Grounding him to reality with his touch, his voice, his mental presence if it was needed (and even if it wasn't) and keeping him sane.
But when the Master wasn't there to comfort him and keep him tied to reality, that's when things got really bad.
It wouldn't be so bad at first. He was used to being alone and dealing with the memories and guilt alone after all. But now that he was getting used to the other Time Lord being around him and helping him through everything, it was getting harder each time. When it started getting to a few months without him around, that's when he would set the controls to take him back to the last time and place he had been with the Master.
And then, he would wait.
The TARDIS tried to help by moving the rooms around so he was closer to the rooms she thought knew he needed, like the kitchen or his bedroom, when he started to forget to eat or he stopped sleeping because sleep brought pain and the memories of him and the Master together. And he couldn't handle it because while he slept he could be happy, but he knew it would end, so he gave himself to the relief it brought and revelled in the warmth. And when he woke up he would be even more broken than before and he couldn't take it. Not without the Master. Without him he was numb.
He knew he was skinny, Donna had reminded him enough times about this fact, so he at least attempted to eat sometimes to make the TARDIS feel better and not to worry the Master when if he returned. But sometimes he just forgot. Sometimes he was just too consumed by all the pain and grief and sadness and loss to notice and he just lost track of the world around him, stuck in his memories of times long since passed.
And then the Master would return.
As soon as the key was in the lock and the door began to swing open, the Doctor was already halfway across whatever room he was in, no matter the size, damn it, because the Master was back. He was back and he had returned for the Doctor. For him. So he ran; he ran as fast as he would if his life was in danger, maybe faster, for the Master, because he already knew who it was and it was his Master. His Koschei.
And he always knew when he was back and with him.
And there he was. Standing in the TARDIS doorway, his smug smirk on with his black suit, tie, and shoes, with a few stray specks of blood decorating the crisp white of his shirt.
The Doctor pretended not to notice them as he passed the controls. The only thing that mattered was that Koschei was back now and he could make all the bad things go away.
He always could.
And he did. As soon as the Doctor was safely in his arms he gave the Doctor's mind a nudge, picking him up as he rested his head on his shoulder, face in his neck, just breathing in his scent and holding onto him as if he'd disappear if he let go. The Master tsked him because really Doctor, did he eat at all while he was away? Because starvation was actually bad for you and unless he started taking better care of himself, he was going to have to think up some creative new ways to feed him.
The more the Master talked while carrying him to the bedroom, the more the Doctor's eyes cleared until he was fully aware of what was going on around him, where exactly he was being taken, and just who he was with.
And when he realised that the Master really was there with him, he began to smile. Finally content.
They reached his room in a matter of minutes. The TARDIS must have known that he needed this, clever girl, and as soon as the Master saw he was fully aware again, the kissing resumed. He could feel himself letting go of all the bad things, all the sadness and guilt, as he returned to himself.
And then Koschei pushed in, physically the same time as he did mentally, and they danced. He didn't even remember his or the Master's clothes being removed or even being placed on the bed. Their consciousnesses swirled around each other in an intricate display of love and desire and need and just pure, raw emotion that moved faster and became increasingly intense as they neared their completion. Fingers burned trails of liquid fire where they roamed over cool skin and angry red marks were left whenever teeth met flesh in a statement of possession.
Theta came first, a cry of his lover's name leaving his lips. Koschei soon followed, the musical lilt of their names joining together even in the air.
They regained their breath after a few moments and, as the Master began to pull out, the Doctor let out a quiet whimper and a murmur of "Don't." The Master stilled. He soon realised what Theta wanted and smirked, pulling him close and wrapping his arms around the taller Time Lord.
"Have you been lonely, my dear Doctor?"
He hates it, hates how vulnerable he's become and how he lets the Master play him like this, but mostly he hates how he's come to depend upon it. How he only feels good and himself again if Koschei is there with him and how he knows that he knows, and that's why he can get away with it. Because he knows Theta will always be there, waiting for him in the TARDIS, so broken, so beautiful. So his.
"You are not alone, Theta."