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
The Doctor (Doctor Who) "definite sense of Englishness". When Hartnell left the programme after three years due to ill health, the role was handed over to character actor Patrick Troughton. , official television productions have depicted thirteen distinct incarnations of the Doctor. The longest-lasting on-screen incarnation is the Fourth Doctor, played by Tom Baker for seven years. Within the narrative, these changes were explained as regeneration, a biological process which heals a Time Lord when their incarnation is about to die. Consequently, the Time Lord is given a wholly new body. In "The Deadly Assassin", the concept of a regeneration limit is introduced, giving