Following a brief glimpse of footage in a new BBC promo that debuted last month, Masterpiece PBS has unveiled a full trailer for the highly-anticipated Sherlock special, which still doesn't have a premiere date in place at this time. Unlike the regular series which is set in present day, this Christmas special transports both Sherlock Holmes (Benedict Cumberbatch) and John Watson (Martin Freeman) all the way back to the 19th Century, where Sir Arthur Conan Doyle's original Sherlock Holmes stories took place. While it isn't known why this crime-fighting duo have been transported to the Victorian Era, we do get to see Sherlock put on a very special hat.

It was revealed at Comic-Con this summer that the Christmas Special will arrive sometime in December, and fans will also get the chance to watch the special in select movie theaters in the U.S. Executive producer Steven Moffat told the SDCC crowd that this is one of the finest episodes they have ever produced, and he even considered shifting the series to this time period, permanently. The first trailer offered a reference to the show's long absence, which hasn't aired a new episode since early 2014, with Benedict Cumberbatch's Sherlock Holmes telling his landlady Mrs. Hudson (Una Stubbs) that "dismembered country squires" are "notoriously difficult to schedule," referencing the actor's own increasingly busy slate of productions.

This special will be followed by the long-awaited Season 4 of Sherlock, which will be comprised of three 90-minute episodes, airing sometime in 2016. It isn't known when production will begin on the new season, but Steven Moffat said in March that the new episodes will be "bloody frightening" and much darker than the first three seasons. No new characters have been confirmed for Season 4 yet, but we could see the return of another familiar face.

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Sherlock Season 3 featured Holmes returning from the dead to watch his best friend John Watson marry Mary Morstan and square off with his ultimate nemesis in Charles Augustus Magnussen. After the briefest of exiles, Sherlock came back only to face one of his biggest mysteries yet. Why is Moriarty's face being broadcast on every television in the land? It still hasn't been confirmed yet if Andrew Scott will return as Moriarty, who was killed off at the end of Season 2, but the actor did make a surprise appearance in a video shown to fans at SDCC.

Sherlock is written and created by Steven Moffat and Mark Gatiss, and inspired by the works of Sir Arthur Conan Doyle. Sherlock is produced by Sue Vertue and the executive producers are Beryl Vertue, Mark Gatiss and Steven Moffat for Hartswood Films, Bethan Jones for BBC Cymru Wales and Rebecca Eaton for Masterpiece. While we wait for more details on this highly-anticipated Sherlock special, check out the new trailer.