When it finally happens, it's an anticlimax. For all his elaborate set-up, decades of waiting, and theatrical evil laughs (even at his worst, he understood the necessity of the occasional evil laugh); for all the Doctor's near-inevitable thwarting of his plans -- Tremas of Traken managed to wander right where he shouldn't. And getting a new body, in the end, is no more than an exercise of will, a sigh, and opening his eyes in a world which is no longer his enemy.
Tremas would have been no-one's first choice, not even poor Kassia's, really, but the man has, it seems, no survival instinct. And commendable taste in clothing. He lets the poor, crumpled, oxidized, entropy-ridden remains of his thirteenth body simply fade back into the oblivion they've been fighting him for, and makes a few minor adjustments to the new body. His body, now.
Such sweet relief to be in a body that wants to function, that wants to live. It takes him awhile to realize that the strange bliss he's feeling isn't a chemical imbalance, it's simply an absence of pain. He had forgotten that such things were possible. He can move without needing a supreme exercise of will; he can move without needing to think about it at all. He had forgotten that it could be a pleasure to breathe.
He wastes probably more time than he should simply reveling in the freedom of proper embodiment. He walks, he strokes the walls and the console of his ever-patient TARDIS, he finds a sink and drinks a glass of water and feels the cool wonder of it travel down his throat, and then he glances up at the mirror.
It isn't a bad face, especially with a few decades removed and some judicious tweaking of the facial hair. Perhaps a bit too reminiscent of certain previous bodies of his, but one does like to stick to the classics. What it isn't is a Time Lord's body. He has made some adjustments already, using the remains of energies of the Keeper, to make it easier to live in; slight rearrangements of the neural pathways that will make it easier to use a Time Lord's telepathic power, the freeing of the temporal senses, a few subtle changes in favor of durability. He could probably do more, at least as long as the Keeper's energies persist in him, erase the last vestiges of Trakenite physiology in favor of superior biodata, and a face that is his own, rather than being just different enough, just coarser enough, to be wrong. He won't be able to grant himself more regenerations; even the Keeper's power can't do that, but he could get a near-perfect match besides that. He stands in front of the mirror, marking out each individual change that will be necessary; it will use the last of the power, but he had no particular plans for it.
But he stops himself right before he lets the power free. Is it, he thinks, truly necessary? Trakenites are, of course, a poor second to Gallifreyans, but there are worse bodies to be trapped in, if he must make do. Besides their longevity and generally disgustingly cheerful good health, Trakenites also have highly advanced mental abilities as primitive hominids go, and this one in particular has long trained his mind to the processing of huge amounts of causal and empirical data in a way that a Time Lord might find quite useful.
No, he convinces himself, standing in front of the mirror, at the face that is almost but not quite his. This body will have its uses, and deviation from the Time Lord norm could quite possibly give him unexpected advantages. At any rate, this is the body that his new autonomic and somatic reflexes are used to. To remove that well-trained synchrony would be wasteful.
The whole Logopolis incident is ... embarrassing. He blames it on being unaccustomed to this body's neurochemical responses. Clearly both his humiliating misapprehension regarding the purpose of the calculation and his completely not humiliating at all response to the Doctor's proximity (and his awareness of the Doctor's response to his proximity) are the result of primitive Trakenite emotional cascades and nothing to do with him; some more time ito adjust and he will be back to his normal equilibrium of supreme competence. Yes.
At any rate, the Doctor's interactions with his newest embodiment are not humiliating at all; they are infuriating. The Doctor ought to furious at him for the death of his friend, or, perhaps, pitying, that the Master has sunk to the level of a common body-thief. Instead they fit together, working on the new programming for the telescope, as comfortably and easily as if they have done this before. The Master holds out a hand for a screwdriver and the Doctor passes it to him wordlessly; the Doctor fumbles and nearly mis-keys a vital switch, but the Master is there before he has a chance to call out. The motions and the body-language between them are far too familiar, and the Master nearly destroys an entire memory-bank when he recognizes the dance. The Doctor is subconsciously responding to him precisely as he had responded to Tremas, working together in his laboratory on Traken. The Master had watched them, avariciously, through the screens in his Tardis, and knowing to look for them, he can replay the same scenes in Tremas's memory: an easy familiarity, with no hint of fear or threat and what he can easily recognize, now that he's in a body with the spare energy for it, as a frisson of (quite mutual) attraction.
He wants to (work alongside the Doctor like this, forever) (own the universe, destroy everything)(have this for himself, not in another man's right) kill them both. And everybody on Traken, for good measure. But he's already taken care Tremas and Traken, so all he can do is watch in disbelief as the Doctor lets himself plummet from the catwalk.
He should leave. He should leave, and find somewhere safely far away from the Doctor, and regroup. Everything else aside, the Doctor's leaking artron energy like a fountain, his mind reaching out uncontrollably and unguardedly for anything familiar it can touch, and with his own new embodiment still not entirely settled, it's dangerous for him to stay in the area. But he can't make himself leave entirely. He watches from the safetly of his TARDIS as this thing he started finishes, feeling slightly nauseous at the sight of the Doctor, lying so still in the scrubby grass, as the Watcher approaches and his old face fades in to a new one. Something in him wants to be sad, to be angry, to bubble up to the surface and stay there.
Something else in him says, with a level of dispassion he though he had lost, Well, most interesting. This one is downright gorgeous, isn't he? I'm suddenly not quite so insulted at being second choice.