"The Timeless Children" is the tenth and final episode of the twelfth series of Doctor Who. It was the final part of a trilogy of related episodes, and was also 65 minutes long, and involved more special effects and more scenery than the average Doctor Who episode.
At the end of the last episode, The Master, who had last been seen in Spyfall, appears to whisk The Doctor off to the wreckage of Gallifrey, to reveal secrets to her. Meanwhile, the companions and remaining humans are back in the real world, fighting or fleeing from the army of Cybermen. Whereas the previous episode showed just how powerless untrained humans are against the technology of the Cybermen, this episode features a more standard Doctor Who take on the matter: despite being hopelessly outgunned, our scrappy humans cleverly evade or fight against the Cybermen for the episode, while the Doctor...
So the main point of this episode is that in a series of flashbacks, The Master reveals the secret history of Gallifrey and the Time Lords, which is a shock to The Doctor, and also to the audience. Because rather than being just a "standard" Time Lord that ran away for adventure, this episode describes how the Doctor was actually an alien child from a different dimension, and that the powers of the Time Lords, such as regeneration, were derived from them. And that, after possibly hundreds of regenerations, the Doctor was mindwiped and became the William Hartnell First Doctor. All of this is revealed in straight exposition and flashback scenes, while the fighting goes on "outside".
Then, in a final climactic scene, the very hammy Master reveals a new master plan: since he has killed all the time lords and has Cybermen handy, he is going to use...Time Lord corpses to reanimate Cybermen into a horde of hybrid killers. A plot that is foiled, at the last moment, by Doctor and companions, ending the series arc, but leaving many questions for the viewer. On one hand, the revelation that The Doctor has had countless regenerations opens up the possibility for the character, and reinforces the idea that the Doctor is something beyond understanding. On the other hand, it undermines the character development the Doctor has had over sixty years. For me, I thought the story was exciting and interesting when I watched it, but when I woke up the next day, I had almost a hangover, wondering what all the gigantic, disruptive revelations in this episode would mean. Would it really change anything, or was it merely an attempt to shock the viewer, with no relevance or substance?
This episode certainly gets points for taking chances, but it has two major flaws: first, it is given in the form of an infodump with the Doctor herself merely a passive audience to the Master's revelation, and second, I am not clear on what this revelation means in terms of character development.