Furthermore, this story hugely contributed to one of the show's most omnipresent topics: the consequences of time travel, specifically meddling with fixed points in history. This story demonstrated one of the consequences of preventing/delaying a fixed occurrence in someone's timeline, with death being used as an example for this episode. It also introduced the idea that one's actions also counted as fixed events and therefore could not be undone. The latter contribution did away with the question of why the Doctor never bothered to go back in time and undo his mistakes. Through these contributions, the show was able to firmly establish some of time travel's cardinal rules, which would clarify many events in previous episodes and set limitations for the rest of the show.