Doctor Who, BBC One (UK) / BBC America (USA)
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The original Doctor Who TV series ran from 1963 to 1989. The new series began in 2005, in the U.K., but I didn't see it until 2006, when it aired in the U.S., on Sci-Fi Channel. (That network would air the first four seasons, before American rights were picked up by BBC America, which had already been airing reruns of seasons 1-4, and then became the primary American distributor of the series from season five onward.) So... Quick recap of the old series: there's this human-looking alien called "the Doctor," who is a Time Lord from the planet Gallifrey. He has a time machine/spaceship called the TARDIS (Time and Relative Dimension In Space). The tardis is disguised as a 1950s British police box (which is like a phone booth), but it's bigger on the inside. He uses it to travel throughout time and space, and has lots of strange and exciting adventures, sometimes even on Earth. Sometimes he travels alone, and sometimes with one or more Companions. The Doctor is centuries old, and occasionally regenerates a new body, so he can be played by different actors. But it's not just his appearance that changes, he also gets a new personality when he regenerates... but the one constant is that he's always a hero (and kind of a rebel). But Time Lords can only regenerate twelve times (under normal circumstances), for a total of thirteen lives. And this new series begins with the ninth Doctor (played by Christopher Eccleston). (Well, we'll call him the ninth Doctor, anyway. He might technically be the tenth, but that's a story for another time. Same rule applies to all subsequent regenerations in the current revival of the franchise.)
Thirteenth Doctor (Jodie Whittaker)
Series Eleven (2018)
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Details coming whenever I get to see this season. But I do want to mention that Jodie Whittaker was announced as the thirteenth Doctor in July 2017 (at which point I still hadn't seen any of the show since the middle of series 6, back in 2011). This would be the first time the Doctor was played by a woman (at least canonically). Predictably, a lot of misogynistic fans were angry about this, while many other fans (I hope it would be accurate to say "most fans") were happy about it. Also this season, Chris Chibnall replaces Steven Moffat as showrunner.