
When the series began in 1963, it introduced the enduring conceit that the Doctor takes normal-seeming humans as “companions” on his adventures, spanning space and time, and fighting weird threats in the present, past, and future. Among the threats were the robotic Cybermen (like Star Trek’s Borg, but decades earlier) and the Daleks, who issued a menacingly shrill chant of “Ex-ter-minate! Ex-ter-minate!” that was only slightly undercut by their resemblance to ventilation ducts outfitted with paint rollers and toilet plungers. When the First Doctor (William Hartnell) left the series due to failing health, producers came up with an unorthodox way of replacing him, one that would become a signature aspect of the show and provide natural jumping-on points for new viewers: regeneration. Instead of just recasting the lead and pretending that it had been that guy all along, Doctor Who makes a big deal of these transitions.

The Doctor dies, usually in a self-sacrificing sort of way, there’s some face-morphing effects, and a new guy (never been a woman, yet, though it gets talked about a lot) materializes in his place, with the same abilities and knowledge (more or less), but a different personality and style. So Hartnell left, replaced by a variety of actors with varying degrees of success, and companions rotated through even more frequently as the role certainly wasn’t as glamorous as playing the enigmatic lead.Īs the show grew more popular, with story lines playing out in novels, radio plays, and video games, the mythology grew more and more complicated, with complex rules developing over the decades. One rule, probably added to make sure that regenerating didn’t seem too much like a stakes-free enterprise, was that a Doctor can regenerate only a finite number of times: specifically, twelve (meaning there can only be thirteen Doctors total).


That must have seemed like plenty when the idea was introduced in the 1970s, but it became a little confining by the time the 21st-century reboot came along. Depending on who you ask (the web will either sort this all out for you or send you screaming into the night), the newest Doctor is the twelfth or the thirteenth.
