The Doctor (Seven Keys to Doomsday)

One incarnation of the Doctor travelled with his companions Jenny and Jimmy to the planet Karn, to prevent the Daleks from gaining the Crystal of All Power. (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, where 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.