Time Lords can ONLY regenerate 12 times ( which has now been exceed by what...2? ) and when The Master tried for a thirteenth ( though they did bring him back for the new series and THAT didn't work at ALL! ), he begins to crumble, after a bit, a la Dorian Grey, at the end of that book and film...turning into dust!
This whole thing is RIDICULOUS! ... Time Lords can ONLY regenerate 12 times
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Science progresses and things change, so its easy to come up with an explanation, and I seen to remember there was something about that in one of the shows - I don’t remember which.
The Master always come back - as he always has backup plans in case the current one fails. In one episode his consciousness was inside a ring, and he managed to takeover the body of the wearer.
The ridiculousness is part of the fun.