Previous Doctor (Seven Keys to Doomsday)

A previous Doctor regenerated into a new incarnation after being attacked on the planet Karn. (AUDIO: Seven Keys to Doomsday)

Behind the scenes

 * In the play on which the audio story is based, the incarnation who regenerates into the story's main Doctor is Jon Pertwee's Third Doctor. The play is not, however, considered a valid source on this Wiki, and as for the audio story itself, Nicholas Brigg seems to make little effort to sound like Jon Pertwee.