The Doctor walks straight out of the door as soon as the TARDIS lands, hardly even taking heed of where. Somewhere with caves. When he crawls back in, wheezing and weak and barely able to stand, the Master is waiting for him, still wrapped in his oversized coat.
“Spectrox toxcemia,” the Doctor manages to choke out. “Synthesise an antidote...”
The Master watches, one eyebrow raised.
As the Doctor falls, the Master is there to catch him, lowering him with surprising care to the floor by the TARDIS console. He rests one hand on each of the Doctor's temples, and shakes his head sadly.
“You closed your mind to me, Doctor. I must know what you're hiding. Even if...”
“You needn't worry, Doctor. You will not be allowed to die.”
Through the blur and whirl and howl of the Vortex, through the white-hot pain of raw energy seething through him, through the wrack and ruin of his body giving up its last, the Doctor can feel the Master there, cold and quiet, dripping down into the fiery recesses of his memory. He clings to the Master's presence, body and mind and soul, as the change comes.
He only half-sees the Master's face swirl and change as well.
When the change has died, rattled down to the very bones of him and left them done, the Doctor sits up at once, pushing the Master away with a strength that surprises him. He's a bigger man – he can feel it in the tautness of the charred frock coat around his shoulders, see it the size of the hands that he holds up to scrutiny. His eyes narrow sharply as he glances at the Master pushing himself upright and sees exactly what he's done to himself. He knows. The Master's mind is open to him, and he knows the Master knows everything now. He has to, to have chosen that face.
Curiously, it feels better now the truth is out. The change seems to have helped. Outrage springs more easily to the Doctor's mind now than anguish ever did, and the first words of his new life are spoken not in guilt, but in anger.
“That face is not yours to wear!”
“It was not yours to keep from me,” the Master snaps, and it is a snap – harsh and cruel, and seeming to invite the barking chuckle the Doctor knows so well. He's taller now – he fits that absurd coat, at least – and the hair and beard are neater, the teeth straighter, the hair pure black. It's him. The man the Doctor allowed to die.
“To steal from a thief can hardly be accounted a crime,” the Doctor snaps back. “And can you honestly say you're proud of what you did?”
“It would have been my finest hour. The universe, held hostage against itself...”
“It was a disgusting piece of improvisation in the face of your own incompetence. It would never have worked.” The Doctor hauls himself to his feet and peers at himself in a screen on the console, admiring the archness of his new face as he continues to lambast the Master. It has a dignity that belies itself – the hair notwithstanding. “And, finest hour or not, it hardly excuses the depths you sank to before. Or afterwards.”
“Evidently I was desperate,” says the Master, as he too stands up, keeping his distance, staying on the other side of the console. “Assuming that your memories of my prior condition are anything to rely on, which I doubt.”
“And what exactly is that supposed to mean?” The Doctor continues, talking over the Master's response as if he isn't there. “I recreated you with the only resources I had available. You could at least demonstrate some gratitude.”
“Gratitude?” A scowl sprawls across the Master's face. “I hardly think gratitude appropriate. You watched me die, Doctor, and then you brought me back and you stole my memories!”
“I offered you salvation,” says the Doctor, creeping around the console, nudging at switches and controls, his hands twitching and flexing. The Master creeps too, in the same direction, keeping the time rotor between him and the Doctor.
“The only salve you offered was to yourself – to your own inflamed conscience.” He chuckles, eyes closing slightly, head tipped back, and the Doctor's hands ball into fists. “And it didn't work, did it? You resurrected me, seduced me, and then found yourself a suitably enthusiastic way to die.”
The Master laughs, and that's too much to bear. The Doctor howls, and charges around the console. The Master tries to dodge, but he's still not used to his body, and he slips. They fall to the hard floor, and the Doctor's hands claw at the Master's throat. Even as the Master brings up his own to struggle, the Doctor reaches the catches beneath the Master's jaw, and flips his face open. A cry of rage and fear and pain dies as the Master's faceplate fades to neutral silver, and the flicker of lights on exposed circuitry is the only sign of operation that remains.
As he rises, the Doctor catches sight of his face in the mirror, contorted with rage that falls away the moment he sees it, gaping horror emerging from behind to settle in its stead. He groans, and stumbles away, through the door and into the bowels of the TARDIS.
“Abominable, abhorrent, benighted soul!”
He's down there for some time, losing himself in the business of choosing a new wardrobe. The Master's coat would fit him, but that thought worries him, drives him along a dark pathway toward what waits in the console room, and he pushes it aside.
He can't concentrate, though, and eventually he picks up something that fits, struggles into it, and considers himself in the mirror. It's hideous, and yet there's a certain something about it; any man who can walk down the street in that coat is afraid of nothing. The Doctor straightens, huffs in his not insubstantial paunch a bit, and makes his way back up the stairs. On the way down, he was jumping at shadows. On the way up, he strides, fearless, armoured in his inviolable resolve.
Striding back over to the Master, he rests a hand on the android's free-hanging face, almost tenderly swinging it back into place. The Master twitches as he comes online, moving through confusion to a snarl.
“I told you never to...”
“I know. And I apologise, from the utmost reaches of both my hearts. But, Master...”
“I will never suffer myself to be judged by you. There is nothing I can do that's worse than what you've done. Do you understand?”
The Master's sneer twists, and he looks away from the Doctor's glare, which becomes a roll of the eyes, accompanied by a low, weary sigh. “I'm not sure I approve of your new regeneration, Doctor. It doesn't become you at all.”
“I'm not asking for your approval.” He places a hand on the Master's chin and turns the android's face toward his own. “Merely your acknowledgement, given in turn and good faith. I am the Doctor, whether you like it or not - and you are, irrefutably, the Master.”