First Doctor: There is good and there is evil. I left Gallifrey to answer a question of my own. By any analysis, evil should always win. Good is not a practical survival strategy - it requires loyalty, self-sacrifice and love. So, why does good prevail? What keeps the balance between good and evil in this appalling universe? Is there some kind of logic? Some mysterious force?

Bill Potts: Perhaps there's just some bloke, wandering around, putting everything right when it goes wrong?