Doctor Who
Thirteen actors have headlined the series as the Doctor. The transition from one actor to another is written into the plot of the show with the concept of regeneration into a new incarnation, a plot device in which a Time Lord "transforms" into a new body when the current one is too badly harmed to heal normally. Each actor's portrayal is unique, but all represent stages in the life of the same character. Together, they form a single lifetime with a single narrative. The time-travelling feature of the plot means that different incarnations of the Doctor occasionally meet. The Doctor is currently portrayed by Jodie Whittaker, who took on the role after Peter Capaldi's exit in the 2017 Christmas special "Twice Upon a Time".
Doctor Who
The Master is the Doctor's archenemy, a renegade Time Lord who desires to rule the universe. Conceived as "Professor Moriarty to the Doctor's Sherlock Holmes", the character first appeared in 1971. As with the Doctor, the role has been portrayed by several actors, since the Master is a Time Lord as well and able to regenerate; the first of these actors was Roger Delgado, who continued in the role until his death in 1973. The Master was briefly played by Peter Pratt and Geoffrey Beevers until Anthony Ainley took over and continued to play the character until Doctor Who's hiatus in 1989. The Master returned in the 1996 television movie of "Doctor Who", and was played by American actor Eric Roberts.