The Doctor (Seven Keys to Doomsday)

One incarnation of the Doctor, following his regeneration, met new companions Jenny and Jimmy. In the TARDIS, they traveled to the planet Karn, where they prevented the Daleks from gaining the Crystal of All Power.

This incarnation of the Doctor resembled a middle-aged man, with a hoarse voice. (AUDIO: Seven Keys to Doomsday)

Behind the scenes
This incarnation of the Doctor originated in the stage play Doctor Who and the Daleks in Seven Keys to Doomsday, in which he was implied to have followed Jon Pertwee's Third Doctor. The stage play version of this character, therefore, was the Fourth Doctor before Tom Baker was cast as the true Fourth Doctor for television. In the audio adaptation, an unidentified incarnation of the Time Lord, played by Nicholas Briggs, preceded him.

The Doctor was played by several other actors in later worldwide performances of the original play, including Michael Sagar in the 1984 production of the play in Porirua, New Zealand. Sagar portrayed the Doctor wearing a checkered suit and sporting a black mustache, giving him an overall look clearly distinct from Trevor Martin's take.