Twelve actors have headlined the series as the Doctor. The transition from one actor to another is written into the plot of the show, as well as the differing approach to the role that each brings, under the concept of regeneration into a new incarnation. The show's premise is that this is a life process of Time Lords through which the character of the Doctor takes on a new body and, to some extent, new personality, which occurs after sustaining an injury which would be fatal to most other species. Each actor's portrayal differs, but they are all intended to be aspects of the same character and form part of the same storyline. The time-travelling nature of the plot means that, on occasion, different Doctors have met each other. Peter Capaldi took on the role after Matt Smith's exit in the 2013 Christmas special "The Time of the Doctor".