Beginning with William Hartnell, fourteen actors have headlined the series as the Doctor; the most recent being Ncuti Gatwa, who portrayed the Fifteenth Doctor from 2023 to 2025.^[2] The transition between actors is woven into the story via the plot device of regeneration. When a Time Lord is fatally injured or weakened by old age, their cells regenerate into a new body, while they retain their memories, their personality changes. The different portrayals function as distinct stages within a single, continuous narrative. Due to time travel, these different incarnations occasionally cross paths.