BBC 'Sherlock' Season 4 air date & premiere: Sherlock and Watson go back in time-appropriate era [video]

"The stage is set. The curtain rises. We are ready to begin," Benedict Cumberbatch as Sherlock Holmes opens the trailer below.

The upcoming special will go back to its time-appropriate era, in 19th century Victorian England where it was originally set by author Sir Arthur Conan Doyle. The trailer portrays London in a foggy and more gothic mood with all the characters decked in period garment and gear. Sherlock is also seen wearing the traditional deerstalker cap and smoking a pipe, something that was imagined of the original detective.

We also see an appropriately mustachioed John Watson (played by Martin Freeman) who convinced Holmes, the most famous wearer of the deerstalker cap, to don it. In which Watson says to Holmes: "You're Sherlock Holmes, wear the damn hat." Instead of Holmes fondly referring to John in his first name, he will be addressed as Dr. Watson instead, the trailer suggests.

Through the various visual imageries and mysterious players, the trailer leaves little clue what the special will be about. All we know at this point is that most of the regular cast is in it and that Sherlock is living in his home.

"The name is Sherlock Holmes and the address is 221B Baker Street," Holmes by Cumberbatch says.

According to LA Times, a clip of the special was aired during this year's San Diego Comic Con and the newly released trailer is the official one. There is no set time and date for when the special will be televised but it is rumored that it will air around Christmas. The outlet quoted show co-creator Steven Moffat saying that the plan was to: "put it on television and you'll be able to watch it."

The Christmas special is a standalone and not connected to the past and upcoming new season.

"The special is its own thing," Moffat said, according to the report by Variety. "We wouldn't have done the story we're doing, and the way we're doing it, if we didn't have this special. It's not part of the run of three episodes."

As for the anticipated fourth season which is allegedly going to air early 2016, Moffat stated that the three-episode season will likely reduce the viewers to tears.

"We have a plan...we practically reduced our cast to tears by telling them about the plan," Moffat explained.