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
K9 (Doctor Who) Jaeger) in the year 5000. K9 subsequently travelled with the Fourth Doctor (Tom Baker) and Leela (Louise Jameson) as a companion of the Doctor in his adventures in time and space until "The Invasion of Time" (1978). In this serial, K9 decides to remain on the Doctor's home planet of Gallifrey with Leela. Immediately afterwards, "Doctor Who" would introduce a second incarnation of K9, played by the same prop; the last scene of "The Invasion of Time" shows the Doctor unpacking a box labeled "K9 Mk II". Although the first incarnation of K9 does not appear again in televised "Doctor