A week after the mind-bending twist and shocking final scene of last Sunday's episode The Lying Detective, here's all you need to know about series four finale The Final Problem...

What time and date is Sherlock series 4 episode 3 on?

The Final Problem kicks off on Sunday 15th January at 9:00pm on BBC1, and runs for 90 minutes.

Who's starring in it?

The cast includes Benedict Cumberbatch as Sherlock Holmes. Of course. Martin Freeman as John Watson (even if he's just playing a corpse – highly unlikely). Mark Gatiss as Mycroft, defo. Una Stubbs as Mrs Hudson, almost certainly. And of course we'll get to enjoy more of Sian Brooke as secret Holmes sister Euros, the master of disguise. Meet the full cast here

What's The Final Problem about?

The concluding episode, the ominously titled Final Problem, is named after the original Sherlock Holmes story in which Sir Arthur Conan Doyle killed Holmes off (before bringing him back due to popular demand). But that was dramatised in Sherlock as The Reichenbach Fall, so this is something different.

Sherlock creators Mark Gatiss and Steven Moffat have described the episode as "absolutely crazy" and a "very very finale finale" but also tell us it won't necessarily be the last we'll see of the show.

It will also reveal more about Sherlock and Mycroft's apparently long-lost sister Euros, who was last seen pointing a gun at John's head and pulling the trigger (although we assume he found a way out of that, since he appears in the episode trailer). Euros looks set to be at the centre of the story though, with Mycroft telling Sherlock in the trailer "Every choice you've ever made, every path you've ever taken, the man you are today, is your memory of Euros." Intriguing...

We'll also be treated to at least one massive explosion and will hopefully discover the identity of the mysterious Sherrinford. The question is, will we see the return of one James Moriarty? We've really missed him...

Sherlock series 4: The Final Problem is on Sunday 15th January at 9:00pm on BBC1