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New trailer for the 'Sherlock' Victorian Special reveals more clues

Watch Benedict Cumberbatch and Martin Freeman as Holmes and Watson, as the pair stumbles upon a murder shrouded in mystery and fog in Victorian London.

Benedict Cumberbatch and Martin Freeman return in the "Sherlock" Victorian Special trailer. Video screenshot by Bonnie Burton/CNET

The Great Detective temporarily leaves the modern London setting of the BBC's "Sherlock" for the Victorian era of Sir Arthur Conan Doyle's original work, as Sherlock Holmes (Benedict Cumberbatch) and Dr. John Watson (Martin Freeman) unlock the secrets of another mystery.

The new "Sherlock" Christmas Special trailer from BBC One, returns viewers to 221B Baker Street, and features horse-drawn buggies, ladies in corsets, men in suits and most prominent of all, Watson's manly mustache.

The special has yet to be given an official title, so it's not clear if the plot mirrors a classic Doyle tale, or if this is a brand-new adventure from the minds of "Sherlock" co-creators Steven Moffat and Mark Gatiss.

What we do see is a distraught woman berate Holmes: "You promised to keep him safe!" Though we have no idea yet, who Holmes was hired to protect.

The dark and bleak trailer does not reveal many clues, but we do see a glimpse of Mary Morstan -- also known as Mrs. Watson (played by Amanda Abbington) -- standing by John Watson's side in the 221B Baker Street apartment.

"We all have a past, Watson," Holmes says in the trailer. "Ghosts -- they're the shadows that define our every sunny day."

The Victorian "Sherlock" Special is rumored to air on Christmas Day 2015 on TV as well as select theatrical releases, however no official date has been given. The only hint we get is "coming soon."

Series 4 of "Sherlock" starts airing in January 2016. "Sherlock" airs on PBS in the US and on BBC One in the UK.