Second Doctor
The Second Doctor is an incarnation of the Doctor, the protagonist of the BBC science fiction television series "Doctor Who". He was portrayed by character actor Patrick Troughton.
Doctor Who and the Daleks in the Seven Keys to Doomsday
Originally, the part of the Doctor was to be played by Jon Pertwee, but the role went instead to character actor Trevor Martin. Martin had previously appeared in "Doctor Who" as a Time Lord in the 1969 serial, "The War Games". In the play, it is stated that Martin's version of the Doctor is regenerated from Pertwee's; this makes him an alternate version of the Fourth Doctor. (The play premiered about two weeks before Tom Baker began his television tenure as the Fourth Doctor).