Doctor Who - Twice Upon a Time, on BBC One (UK) / BBC America (USA)
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Caution: spoilers!

This is the thirteenth Christmas special of the revived Doctor Who series. It first aired on Christmas 2017, but I didn't see it until July 2019. It takes place between series 10 and 11. The Doctor finds himself at the South Pole, and he is refusing to regenerate. He soon crosses paths with the first Doctor (played by David Bradley, who had previously appeared in the 2013 docudrama An Adventure in Space and Time, where he portrayed William Hartnell- the actor who originally played the first Doctor in 1963-66). The first Doctor is himself refusing to regenerate, which would mean permanently dying... and none of his regenerations would have ever existed. The twelfth Doctor doesn't remember this having happened, and of course he wants his original self to reconsider. Also of course, the first Doctor has trouble believing that this other Doctor is actually his future self.

Meanwhile, in 1914, a British captain is facing off with a German soldier, when suddenly the Captain disappears, and finds himself at the South Pole, where he encounters both Doctors. They all go into the current Doctor's Tardis (the look of which the first Doctor doesn't care for). But before long, the Tardis is taken into some sort of spaceship, which belongs to a beings from far in the future. On the ship, the twelfth Doctor is reunited with Bill Potts, though he doesn't let himself believe it's really her, since as far as he knows, she should be dead. She gives a very brief explanation about that, something the audience was already aware of from the series 10 finale, but the Doctor wasn't. So it sounds to us like she's telling the truth... but beyond what we already knew, she provides no further information, which I found somewhat suspicious. (Or at least frustrating; I really do want to know more about what happened to her between the series finale and this special. I assume she had more adventures without the Doctor, but maybe I'm wrong.) Oh, I also wanted to say that from the very first time I saw the Captain on the battlefield, I assumed he was a younger version of Brigadier Lethbridge-Stewart, so I found it odd that the twelfth Doctor didn't recognize him. But I was wrong about his identity. (Really, I should have given more thought to how much time passed between the first World War and the Brigadier's first appearance on the show, in 1968. But it's all ancient history, to me.) However, I was not far wrong.

Well, after a little while, both Doctors, and Bill, and the Captain, escape from the alien ship and take off in the first Doctor's Tardis. I don't want to tell you anything about the nature of the aliens or what they want or whether Bill is really Bill. And I really don't want to say much more about the plot of the special. In fact, I'm not sure how much sense it actually made. But I quite enjoyed it, anyway, for both the drama and the humor. (Seeing just how outdated the first Doctor's ways of thinking and speaking were provided a lot of the humor, even if it was kind of cringey. Seriously, in retrospect, it's kind of shocking how sexist a freaking Time Lord- who is not the Master- was, and very much a reminder that the character was a product of Earthly writers of the era. But seeing the twelfth Doctor react to it was totally worth it.) Anyway, they eventually return the Captain to his proper time and place... and one of the things I love most about this special is also one of my favorite aspects of modern history, and the thing that gave the special its only tie-in to Christmas. And of course, the first Doctor decides to regenerate. And... the twelfth Doctor's memory of Clara is restored, in a way I don't want to spoil. But that was nice, even though by now I had totally forgotten that he had lost his memory of her in the first place. And in the end, the Doctor is once again alone in his Tardis, and regenerates into the Thirteenth Doctor. But she (yes, she; love it or leave the fandom, okay?) Ahem. She soon falls out of the Tardis, in midair, and the Tardis dematerializes as she falls toward an uncertain fate. Hell of a cliffhanger, no?