The Doctor (Seven Keys to Doomsday)

One incarnation of the Doctor, following his regeneration, met new companions Jenny Wilson and Jimmy Forbes. In the TARDIS, they travelled 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
Trevor Martin's Doctor originated in the 1974 stage play Doctor Who and the Daleks in Seven Keys to Doomsday. The play debuted after TV: Planet of the Spiders and directly before TV: Robot, and as both Jon Pertwee and Tom Baker were not available for the production, Martin was chosen to play a brand-new incarnation of the character.

The play began with Martin wearing a wig and costume indicating that he was the Third Doctor, not unlike Seventh Doctor actor Sylvester McCoy's portrayal of the Sixth Doctor in Time and the Rani. He would then collapse on stage, and two "audience members" would rush from their sets to help him. An image on a rear-projected screen would then show Pertwee's face turning into Martin's, indicating that he was the Fourth Doctor. As the lights turned back on, Martin would be wearing a new costume with his natural hair. As he awoke, it was revealed that the audience members were his new companions, Jenny and Jimmy, who had been sitting in the audience as a trick of immersion.

In the audio adaptation, an unidentified incarnation played by Nicholas Briggs precedes Martin instead.

The "Seven Keys" 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 chequered suit and sporting a black moustache, giving him an overall look clearly distinct from Trevor Martin's take.