The Doctor (Doctor Who) footage. Tom Baker did reappear, but as "the Curator", an ambiguously different character who he was not credited for playing. A voice actor, John Guilor, recorded a line of audio impersonating the First Doctor, for which he was credited as "Voice Over Artist". In other multi-Doctor stories, the multiple actors are all credited as "The Doctor", the exception being "The Three Doctors" (1972–73), which credited William Hartnell, Patrick Troughton and Jon Pertwee as "Doctor Who" as the 1972 serial preceded the practice of crediting the character as "The Doctor". In "Human Nature" (2007), the plot involves the Tenth Doctor altering
Enlightenment (Doctor Who) serial "Mawdryn Undead", and concluding with "Enlightenment"), the enemy was the Black Guardian, who was last encountered by the fourth incarnation of the Doctor at the conclusion of The Key to Time saga in the 1979 serial "The Armageddon Factor". The story also saw the return of the White Guardian, who had also not been seen since 1979. After penning a number of radio and TV scripts, including episodes of "Crossroads" and "Waggoner's Walk", Barbara Clegg submitted a story idea to "Doctor Who" script editor Eric Saward, an acquaintance from the BBC drama department. Interested in writing for the series,