Anthony Steven made slow progress writing this serial. To the mystification of the production team, he began explaining away his problems with increasingly bizarre excuses - most famously, a claim that his typewriter had "literally exploded"! He was unable to finish it due to illness and as a result, Eric Saward had to salvage the script.
Only the second time the Doctor has regenerated before the end of a season. The first time was when the original Doctor regenerated into the second in The Tenth Planet: Episode 4 (1966).
Colin Baker and Nicola Bryant had a frosty start to their relationship. Bryant had been nervous about suddenly becoming the senior member of the regular cast, and Baker had mistaken this for aloofness on her part.
The Doctor accuses Peri of being a "Peri" a fairy from Persian mythology. In the myth, a "Peri" is a beautiful and benevolent supernatural being or fairy.