The climax to Episode Two is more proof that this new vision of Doctor Who was going to take more chances than ever before. It also appeared that it was going to put The Doctor in more situations that he couldn't immediately get out of. Case in point: rapidly aging The Doctor 500 years so that he appears as an old man, at the very end of his regeneration. Not only can The Doctor not reverse the effects of the generator, but he is too weak to quickly come up with a solution.
The end result of this is not only a more interesting story on account of a more vulnerable hero, but a dynamite performance of this old Doctor by Tom Baker. Baker doesn't play an old Doctor, he plays his Doctor as an old man - still in possession of all his vim and vigour, but with nowhere near the energy. It's a brilliant turn by Baker (and an equally brilliant make-up job) that proves that despite whatever discomfort in his new situation was giving him off screen, Baker was more than capable of delivering the goods when the cameras were rolling.
Baker's performance isn't the only good performance in this, though, as Adrienne Corri as Mena and, especially, David Haig as Pangol do superlative jobs in bringing their characters to life, as well.