I've also been bothered by some of the things you point out about lack of explanation, particularly when and why does the Doctor decide that the world is going to be destroyed? Presumably he wouldn't have left at the end of episode two if he thought it was going to destroy the world, yet by episode four he seems to believe just that. That said, I think the story does triumph over its lack of explanation. Things make a kind of associative sense even if they don't make actual sense and I've long felt Doctor Who needs conviction and aesthetic coherence more than strict logic or scientific accuracy. The excellent direction (perhaps surprisingly good, given that original director Douglas Camfield dropped out partway through due to health problems) and the disconcerting incidental music (the only post-1960s story with a score drawn entirely from stock music, which perhaps makes it seem stranger) give the whole thing a disturbingly oppressive feel, especially as the evil alternate Earth (possibly Stalinist rather than Fascist - the script seems to deliberately play it both ways) feels brutal in a way that Doctor Who rarely manages . Just compare with the next fascist Earth, in Day of the Daleks, which is a good story, but a little bit more kids' TV. The real problem is, as you said, the final episode. Having destroyed the world, where do you go next?