Sherlock may not have ended its second Season 4 installment with a major death (at least not one we saw), but boy, if “The Lying Detective” didn’t re-contextualize the year’s major threat. Our first trailer for Season 4 finale “The Final Problem” confirms [SPOILER]’s survival, but does our new big bad mean [OTHER SPOILER] is gone for good?

You’re warned of full Sherlock spoilers for Season 4 midpoint “The Lying Detective” from here on out, but it seems Moriarty may not have been the one behind those posthumous “Miss Me?” messages after all. Instead, Sunday’s latest revealed that the mysterious “Sherrinford” is none other than (or at least claims to be) Sherlock and Mycroft’s “secret sister,” Eurus (Siân Brooke).

Eurus invoked the “Miss Me?” in an invisible ink message to Sherlock, though it remains unclear if she was the one behind the first Moriarty message, or how much Sherlock and Mycroft know of her. In any case, so reads the synopsis for next Sunday’s Season 4 finale “The Final Problem,” whose title references the famous (and deadly) showdown between Sherlock and Moriarty:

In the final episode of this new series, written by Mark Gatiss and Steven Moffat, long buried secrets finally catch up with the Baker Street duo. Someone has been playing a very long game indeed and Sherlock and John Watson face their greatest ever challenge. Is the game finally over?

Benedict Cumberbatch returns as Sherlock Holmes, with Martin Freeman as John Watson, Mark Gatiss as Mycroft, Rupert Graves as Inspector Lestrade, Una Stubbs as Mrs Hudson, Amanda Abbington as Mary Watson, Louise Brealey as Molly Hooper.

There’s no telling yet why Cumberbatch and producers claim Season 4 reaches a sort of natural endpoint, but what clues are hidden in the finale trailer?

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