"Twice Upon a Time" is the 2017 Doctor Who Christmas Special, and was the final episode of the Twelfth Doctor portrayed by Peter Capaldi, and of executive producer/showrunner/head producer Steven Moffat. It is a "multidoctor" episode, featuring the Doctor meeting his first incarnation, here played by David Bradley. (Bradley had previously played William Hartnell, the original actor of the First Doctor, in the 2013 documentary An Adventure in Space and Time, and here he plays the First Doctor proper, rather than his actor playing him. It also features the regeneration of the Doctor into their thirteenth form, portrayed by actress Jodie Whittaker. This is the Doctor's first regeneration into a woman, and the publicity (and pieces of controversy) surrounding this caused much anticipation for this episode.
I know that for someone who is not familiar with Doctor Who, that last paragraph probably was hard to follow. And, for that matter, for people who are familiar with Doctor Who, as well. The episode starts off slowly, at least. After the events of the last climactic story, the Twelfth Doctor is dying and trying to stop himself from regenerating: he doesn't want to go through the difficult process again. In the snow in the south pole, he is greeted by another lost man, who turns out to be his younger self, also on the verge of regenerating, and also trying to avoid the process of change. And then time stops because some aliens are trying to kidnap a dying First World War Doctor, who stumbles into both of the Doctors.
Remember what I just said about Doctor Who being kind of hard to follow? I am saying it again. Especially since all of these plot elements aren't really that important---this episode exists more as a farewell to the show's current incarnation than a plot driven vehicle.
Inside of a cathedral like space-ship, the Doctors meet the current Doctor's past companion Bill Potts (played by Pearl Mackie), but since the Doctor last saw her dying, he thinks it is a duplicate, and must unravel what he assumes to be an evil plot, as well as finding out why the World War I soldier is being targeted by time travelers. What all of this does is leave the Doctor with a new understanding of his role, and he finally decides to regenerate, leading to the last two minutes of the episode, where Jodie Whittaker is introduced, in an introduction that left me waiting for her first full episodes, next autumn.
Okay, this is going to be my last apology for Doctor Who. It is a great show. It just gets bogged down sometimes, and its hard for a fan to figure out what to explain to someone not familiar with the show. Ignore the plot stuff, let me explain this episode in context.
This episode was full of Moffat's trademarks, both visual (transparency, mirrors, liquid) and thematic (memory, identity, paradox, duplicates). All of these made Moffat a prodigy as a writer on Doctor Who, before he was in charge of the show. I have compared Moffat's writing to frosting: it certainly is a fun treat, but it is hard to eat nothing but it. Moffat's conceptually complicated stories were refreshing and exciting a decade ago, when Doctor Who's revival was just getting started, and when the very idea of internet fandom was still spreading to somewhat new demographics. (Before the revival, the idea of Doctor Who's fans being predominantly young women would have been unexpected). Moffat's stories were witty, complicated, intriguing and thought-provoking. But after the 20th story where someone realized that they weren't really themselves and that the world around them was a paradox created by powerful entities with one singular, exploitable flaw, the ideas started to drag. I can't fault anything in this episode: as usual, the acting was superb, the design and art were incredible, and the plotting was well-crafted. It is just, as the story says, you either accept that you change, or you die.
Which brings us to the most important part of this episode: the introduction of Jodie Whittaker. In general, most people welcomed Whittaker, a talented but not overexposed actress who has worked in both drama and comedy for the role. There were obviously some objections to the Doctor turning into a woman. For my part, I welcome the change, but not merely because it is a change in gender. If the production team was just going to cast Whittaker's doctor as another version of David Tennant's tenth doctor, a wacky young person one-lining their way across the universe, then the gender change will be nothing but a trick. Doctor Who is a series that the writer can do anything with: we could have an entire series where the Doctor is amnesiac and has forgotten most of their powers. An entire series of costume dramas with minimal science-fiction elements. Linear adventure serial stories. Slapstick comedy. As much as I respected "Twice Upon a Time", the end result was me waiting eagerly to see what direction new show-runner Chris Chibnall and Jodie Whittaker will take the show.