Predictably, this peek gives us almost nothing to go on in figuring out what the heck it is. Some are saying it’s from episode 5 or 6 (which may or may not be a two-parter), but as far as I know, they’ve only said it’s from Block 3 (which would include those episodes). So, what the heck is this?
Regarding the may-or-may-not-be-a-two-parter, the announced titles for 5 and 6 are “The Girl Who Died” and “The Woman Who Lived”, a pairing which practically screams “two-parter”. And they’re both period pieces — the same period, evidently, with shared location work. But here’s the weird part: they have different scriptwriters, which is really unusual for a two-parter. (They do share a director, but this could be purely for practical reasons, as it would permit more efficient location work.)
So what’s up? Is Moffat trolling us? Well, given that he shares writing credits for the first of the two, that seems very likely. There’s definitely something being teased and something being hidden about this, but the production team have gotten very good at keeping secrets hidden. I suspect we won’t find the answer until broadcast, and isn’t that exciting?