"Everything ends." So the Doctor says to Clara, and so it does.
I don't follow the logic of having regeneration episodes on Christmas (Easter, sure, if one doesn't mind a little obvious Christian symbolism), which is supposed to be a festive time of year and a program may be watched by family members who don't normally follow the show. Those who watch are left either depressed or bewildered, depending on whether they fall into the camp of regular or casual viewers. Nonetheless, once again we have a Doctor's swan song placed in an episode shown at Christmas.*
Continuing the story line from "The Day of the Doctor," in which Gallifrey was not destroyed but instead placed in a pocket universe, the episode opens with the TARDIS in orbit around a planet - along with Daleks, Cybermen, Sontarans, and more - trying to identify the meaning of a signal sent from the planet. The Doctor's old friend, Tasha Lem, Mother Superioress of the Church of the Papal Mainframe, with its cadre of Silents as confessors, has put a force field around the planet to prevent anyone else from landing there, and she sends the Doctor to find out what is going on.
Once on the planet, which, naturally, turns out to be Trenzalor (in an earlier time than "The Name of the Doctor"), the Doctor and Clara find themselves in a picturesque town called Christmas. The signal is emanating from our old friend, the crack in the universe. Translated, the signal turns out to be from the Time Lords, asking the "one question that may not be answered," the Doctor's true name. (Pause for another "Doctor Who? joke") If he speaks it, the Time Lords will take that a signal that it's safe to return from their pocket universe. If he does so, however, the combined forces in orbit will attack, resuming the Time War. If the Doctor leaves, the forces will also attack the planet to prevent the Time Lords from ever returning. To prevent this, the Doctor stays on Trenzalor, stopping any invaders but unable to leave.
Centuries pass. The Doctor ages. Clara, sent away in the TARDIS for her protection, only to return 300 years later (by the Doctor's timeline; only minutes, one presumes, by Clara's). He tells her he can't regenerate, as this is his 13th and last incarnation; Ten regenerated into the same form and the War Doctor counted as well. The Daleks attack the Papal Mainframe and convert Tasha Lem into a humanoid Dalek (a la Oswin Oswald in "The Dalek Asylum"), then trick the Doctor into leaving the planet to visit Lem. She fights off the Dalek inside her just long enough for the Doctor and Clara to escape and return to Trenzalor. The Daleks remove the force field around the planet and attack. The Doctor appears withered and beaten, ready to accept his death. Clara pleads through the crack for the Time Lords to help the Doctor, and they respond by giving him another set of regenerations. He uses the regeneration energy to destroy the Dalek fleet before retreating into the TARDIS. Clara finds him, he says his farewell to her, then regenerates.
This was no "Day of the Doctor" in terms of plot, either in terms of the big picture (why did the Time Lords appear where they did (or why did the crack appear where it did)? couldn't the Doctor have used the TARDIS to relocate the town of Christmas, leaving the Time Lords where they were until he could find another way to liberate them? and since when did regeneration energy have the ability to destroy a Dalek fleet?) or in terms of the details (the holographic clothing joke had no plot purpose; the brief appearance of the Weeping Angels was pointless; the relationship with Tasha Lem seemed to come out of nowhere; the Cyberman head "Handles" had no purpose other than to provide information at convenient times; and the pacing of the episode seemed off).
Still, there was much in the episode to like. The fable-like narration of the story, set in a fairy-tale-like town called Christmas, set the tone for the Doctor's epic wait. How bored must he have been, waiting for centuries on a primitive planet, the man who couldn't wait around with his friends Amy and Rory for a few days in "The Power of Three"? Clara begging the Doctor to come to Christmas dinner with her family as her boyfriend and the Doctor, naturally, behaving entirely inappropriately was terrific, and I enjoyed the ongoing gag with the Christmas turkey. Seeing Amy in the TARDIS, even as merely a hallucination, was satisfying, and continued the tradition of Doctors seeing visions of old companions (or, in the case of Ten, seeing the companions themselves) before a regeneration. And, in a nod to the classic series, the Doctor helps Handles translate the Time Lords' code by producing the seal of the High Council of Gallifrey, something he "nicked off the Master in the Death Zone" in "The Five Doctors" (and something I hadn't recalled until someone with a sharp memory pointed it out afterward).
Matt Smith, as usual, was amazing, portraying not only a resolute young (-ish) Doctor but also the Doctor as an old man. Foreshadowing his death, he tells Clara sadly that "Everything ends," but at the end he also tells her that he will remember every moment.
Although I'm looking forward to seeing what Peter Capaldi's Doctor will be like, I'm more than a little sad at the end of the tenure of Smith. His portrayal of the Doctor as someone both young and unfathomably old, playful and naive yet a commanding presence, was marvelous in every episode, even ones, like "Time of the Doctor," where the material didn't always measure up to the actor.
* It's true that the Ninth Doctor regenerated in "The Parting of the Ways" in June 2005, and the Christmas episode was the first to feature the Tenth Doctor, though the show recapped the regeneration at the beginning of "The Christmas Invasion." And the end of the David Tennant era came not in the Christmas episode, "The End of Time, Part 1," but a week later, on New Year's Day 2010. Pish-tosh, I say. Both are associated with Christmas.