There are only a few Doctor Who stories which are really direct sequels, but "Terror of the Autons" occupies an interesting space. It's a reprise of "Spearhead from Space", the third Doctor's inaugural story, not just in reintroducing that story's villains, but also in accomplishing another tweak to the show's format. It gives the Doctor an adversary, the Master, an evil Time Lord who's just about his intellectual equal. That alone guarantees its significance, even if the story sometimes plays like a dry run for the season to follow; it ends up being fairly effective, but takes a while to get to its payoff.
