Benedict Cumberbatch and Martin Freeman have been playing very modern versions of Sherlock Holmes and Dr. John Watson in "Sherlock," but a special upcoming 90-minute episode will take them back in time to the Victorian era of Sir Arthur Conan Doyle's original stories.

Entertainment Weekly talked to "Sherlock" co-creator Steven Moffat before a SXSW conference, noting that fans had seen the show's two stars dressed in 19th century clothes during recent filming. Were the characters dressing up for a party, a dream sequence, or some other reason yet to be explained?

"The special is it's own thing," Steven told EW. "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. So we had this to do it – as we could hardly conceal – it's Victorian." He said the standalone episode is its own little "bubble" and it's basically completed.

That episode is expected to debut around Christmas 2015, before the launch of Season 4's three new episodes.