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That was so damn good. My jaw damn near hit the floor just before the credits scene and there were at least three more "holy shit!" moments before the end of the episode. Absolutely loved the hell out of this episode.

Also, it was nice to see Moffat keep his word about The Mistress returning with no obvious explanation (although the way she and Clara disintegrated when they were shot looks very familiar). I appreciate that as a throwback to Anthony Ainley returning from certain doom at least twice per year in the 80s.