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) of a previous incarnation sustaining mortal injury, though he can regenerate from old age and was once forced to regenerate by the Time Lords. A common side effect the Doctor frequently experiences is a period of instability and partial amnesia following regeneration. Some post-regeneration experiences have been more difficult than others. In particular, the Fifth Doctor began reverting to his previous personalities and required the healing powers of the TARDIS's "Zero Room" to recuperate ("Castrovalva"). The Sixth Doctor experienced extreme paranoia and flew into a murderous rage, nearly killing his companion ("The Twin Dilemma"). The Eighth Doctor experienced amnesia due