The revelation of who the mastermind -- who goes by "The Architect" -- really is is a rather predictable and ham-handed affair. They all but tell you the secret halfway through the episode, and by the time of the "twist," it really couldn't have been anything else. All in all, it was a pretty weak trick to drive home the long message of the episode.
What "Time Heist" is looking at is how well we confront ourselves when forced to examine who we are. This is explored from several inventive angles, everything from a shape-shifting mutant to a clone side arc that actually was an unexpected surprise. As in "Into the Dalek" and to a lesser extent "Robot of Sherwood," the Twelfth Doctor seems to be working through a terrible fear of self-examination. He's afraid to look himself in the eyes and see what's there.
Taking this internal journey as a whole in the past five episodes, it hints at something larger. It makes me wonder if the Twelfth Doctor has lost the thrill of victory his predecessor experienced in orchestrating the events of "The Day of The Doctor" and is now wondering whether Time Lords can really be trusted to return to the universe. It's an interesting question. Hints from "Night of The Doctor" do not show much enduring love for his own people by The Doctor. Certainly, Ten remained bitter over the machinations of Rassilon to the end. Before the hunt for Gallifrey can really start, The Doctor is on his own hunt for confirmation of himself as a good man.
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