My review of the previous episode: The Eaters of Light.
Holy crap.
You know, there's a part of me that hates this type of episode. It's the first of a two-parter, and since this is Steven Moffat, I really doubt that he'll be able to pull off a safe landing, especially to a story as ambitious and twisty as this one. But boy, does he make me hope (and that's hard to resist).