This episode contains the first reference to what will become the Time Lords' non-interference policy. Although the Doctor never mentions it directly, he clearly shuns those who meddle with time. As in The Space Museum, time is shown to be theoretically malleable, in contrast to the view of time presented in The Reign of Terror. The concept of time would later be expounded upon to incorporate both elements in numerous later stories, including Attack of the Cybermen and The Waters of Mars (2009).
This episode was watched by 7.7 million viewers on its original transmission.