This marked the only time the Doctor worked alone, with no companion or assistant (for the main action Midnight (2008), he was also without a companion, however Donna did cameo at the start and end). On the other hand, a 1965 story (subsequently wiped by the BBC) called Mission to the Unknown (1965) didn't feature the Doctor or his assistants at all.
With the departure of Elisabeth Sladen as Sarah Jane Smith, Tom Baker told producer Philip Hinchcliffe that he could hold the show on his own and didn't need a companion. With this story already in place, Baker was given a story without a companion as a one-off. However, Hinchcliffe deemed that a companion was a necessary feature of the series and Louise Jameson was introduced as Leela in the next serial.
This serial prompted Mary Whitehouse's strongest letter of complaint about Doctor Who (1963), in which she called it "shocking", "vicious", "sadistic" and "permeated with violence of a quite unacceptable kind". She accused the programme makers of ignoring the BBC's Guidance Notes on the portrayal of violence on television and being engrossed in their own expertise.
Following the complaints about violence in this serial and several other serials produced by Philip Hinchcliffe over the previous two seasons, the BBC's management decided to move Hinchcliffe to the new adult police series, Target (1977), at the end of this season and install a new producer for the next season of Doctor Who (1963), Graham Williams, who was ordered to take out anything graphic in the depiction of violence.
This story was inspired by The Manchurian Candidate (1962), as well as conspiracy theories surrounding the assassination of John F. Kennedy and Bobby Kennedy.