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 Briggs seems to make little effort to sound like Jon Pertwee. As for Briggs himself, it is perhaps noteworthy that he portrayed an unlicensed incarnation of the Doctor in several other audio plays, namely the Audio Visuals.
 * Briggs also portrayed another "previous Doctor" in a licensed production, this one a native of a parallel universe, who regenerated into a Doctor played by Arabella Weir. Additionally, Briggs (putting on a Northern accent) stepped in for Christopher Eccleston as the Ninth Doctor in The Ninth Doctor Chronicles, and narrated extracts from an unspecified incarnation of the Doctor's Five Hundred Year Diary in a 2021 webcast.