A free-standing episode to keep tempers up between seasons should not be a surprise in a series that has successfully combined classic whodunit with modernization for a couple of years now. (Still, it took me a while to get round to 'The Abominable Bride'.)
The new take in the narrative is some bold hopping between past and present, and adroit blending of historic events with the inevitable murder mystery.
As per usual, many of the supporting characters are charmingly ambiguous. There is a real thrill to not quite knowing what to think, and it's all too rare in culture! (And way too common IRL.)