The ethics of time travel; the Doctor wondering if he had the right of changing History in such a big scale in a scene which gave me shivers... It is pure Doctor Who!! I also love how the political commentary is present in all of these episodes: the Kaleds have the entire nazi aesthetic, and the Davros ""supremacist"" mind don't allow any other understanding of what the story is trying to tell us... I only hope that the people who say that "politics" is something to put out of Who would watch those episodes...