 The episode marks the first time the Master has been shown undergoing regeneration; previous serials have shown two natural incarnations of the Master (the incarnation played by Roger Delgado opposite Jon Pertwee's Doctor, and the final incarnation first seen in "The Deadly Assassin" (1976), and subsequently appearing in "The Keeper of Traken").In "The Keeper of Traken" (1981), as a way of extending his life beyond the end of his final natural incarnation, the Master appropriates someone else's body; he does this again in the 1996 TV movie.In the following episode, "The Sound of Drums", the Master states that the Time Lords resurrected him to serve as a soldier in the Time War, presumably giving him a new body with a new set of regenerations (which had previously been alluded to in "The Five Doctors").