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 (series 10)
Preceded by a Christmas special in December 2016, "The Return of Doctor Mysterio", the series is the third and final series starring Peter Capaldi as the Twelfth Doctor, an incarnation of the Doctor, an alien Time Lord who travels through time and space in his TARDIS, which appears to be a British police box on the outside; Capaldi announced in January 2017 that he would be stepping down from the role after the tenth series. The series introduces Pearl Mackie as Bill Potts, the Doctor's newest travelling companion, and also features Matt Lucas as Nardole, who appeared in the 2015 and 2016 Christmas specials. Michelle Gomez and John Simm return as their respective incarnations of the Master. The main story arc for the first half of the series revolved around the Doctor and Nardole occupying themselves as a university professor and assistant while they guard an underground vault containing Missy, who the Doctor has sworn to protect for a thousand years after her supposed execution. Missy later travels with the team in the TARDIS, and eventually partners with her previous incarnation as they all battle a Cyberman onslaught aboard a Mondasian colony ship.