The Third Doctor is an incarnation of the Doctor, the protagonist of the British science fiction television series Doctor Who. He was portrayed by actor Jon Pertwee. Within the series' narrative, the Doctor is a centuries-old alien Time Lord from the planet Gallifrey who travels in time and space in the TARDIS, frequently with companions. At the end of life, the Doctor regenerates. Consequently, both the physical appearance and personality of the Doctor changes. Preceded in regeneration by the Second Doctor, he is followed by the Fourth Doctor.

The Second Doctor is an incarnation of the Doctor, the protagonist of the British science fiction television series Doctor Who. He was portrayed by actor Patrick Troughton. 53 of his 119 episodes are missing.