Sherlock Season 4 airs in 2016, after a one-episode Christmas special to “completely” solve the mystery of Moriarty’s return from the dead

BBC One announced that it will be airing a single-episode Christmas special of Sherlock in December 2015, followed by the premiere of Season 4 in early 2016. Filming begins this month in fact. "It's taken a little while to get the dates sorted as none of the boys are exactly sitting back twiddling their thumbs but there was unanimous goodwill to make this work, so we're thrilled that 221b is going to be inhabited again," said Hartwood Films executive producer Sue Vertue in a BBC One media release.

To fans of the British TV crime drama, "221b" is of course 221B Baker Street, the address of the London flat shared by Sherlock Holmes (Benedict Cumberbatch) and John Watson (Martin Freeman). The crime-fighting duo will be back and so will likely the rest of last season's cast - landlady Mrs. Hudson (Una Stubbs), DI Greg Lestrade (Rupert Graves), pathologist Molly Hooper (Louise Brealey), Watson's new wife Mary Morstan (Amanda Abbington) and definitely Sherlock's brother Mycroft (Mark Gatiss, who is also executive producer, co-creator and writer for the show).

Series co-creator Mark Gatiss told Radio Times that the Christmas special will "completely" solve the mystery of arch criminal Jim Moriarty's (Andrew Scott) apparent resurrection. In Season 2's finale, he was seen killing himself to force Sherlock to also commit suicide. However, in the cliffhanger final scene of Season 3, his face was shown being broadcast across the country saying, "Did you miss me?"

Asked how the upcoming fourth season compares with the Emmy-winning third season, Steve Moffat, Gatiss' co-creator, in an interview with Entertainment Weekly, teased, "We have a plan to top it. And I do think our plan is devastating. We've practically reduced our cast to tears telling them the plan ... we're probably more excited that we've ever been about Sherlock. We're going to try to take it somewhere we've never been before - and I don't mean outer space. It will be a challenging place to go. We always start with a re-introduction that builds toward a darker climax - we might just be going darker [from the outset]. We got a very, very good set of ideas."