This serial includes the first time the word "regeneration" is used to describe the Doctor's transformation.
The Twelfth Doctor, Peter Capaldi, had his letter in praise of this serial published in the Radio Times in 1974.
Jon Pertwee's swansong was originally The Final Game, which would have also seen the end of the Master. Drawing on Barry Letts' proclivity for Eastern philosophy and Forbidden Planet (1956), it would have revealed the Doctor and the Master to be two facets of the same person, with the Master as the "id" (instinctual needs and desires) and the Doctor as the "ego" (conscious perception of and adaptation to reality). At the adventure's climax, the Master would have perished in an explosion, in the process saving the Doctor and others from death - although it would have remained ambiguous as to whether or not this was an act of redemption. It was abandoned following Roger Delgado's death.
This episode was watched by 10.1 million viewers on its original transmission.
Fans voted this number 85 in a countdown of the 163 Doctor Who (1963) stories in Outpost Gallifrey's 40th anniversary poll in 2003.