Remember those pictures that appeared online a short while back, showing Sherlock stars Benedict Cumberbatch and Martin Freeman in period costumes?

Cumberbatch wore Holmes' traditional deerstalker hat and cloak, while Freeman sported Dr. Watson's bushy mustache and a bowler.

Turns out, there was a good reason they were all dolled up in retro garb -- and it wasn't for undercover work.

Entertainment Weekly says that the 90-minute holiday episode of Sherlock will be set in Holmes' classic time period of Victorian London. This frees it from continuity with the rest of the series, and represents a one-off concept piece.

Co-creator Steven Moffat explained to EW that an episode set in the 19th Century is something he and co-creator Mark Gatiss have wanted to do for a long time. The show's first Christmas special, which will air separately from Season 4's standard three movie-length episodes, afforded them the opportunity to go for just such a creative leap.

"It had to be a separate entity of its own," said Moffat. "It's kind of in its own little bubble."

So what might a Victorian London episode of Sherlock be like? And which of Arthur Conan Doyle's stories was chosen for this treatment? Presumably, it would be one that wouldn't make sense to do in any other way. In other words, an episode that doesn't lend itself to modernization as easily as other stories have. On the other hand, it could be a completely original story, not based on anything written by Doyle. Or it could be one of the stories Sherlock has already done, but retold in a 19th Century setting with some cheeky alterations.

Whatever Moffat has up his sleeve -- and we suspect it's a lot more than just a "standard" episode set way-back-when -- you can count on there being plenty of winks and riffs on stuff from the three seasons of the show that have already aired.

PBS, which airs Sherlock in the U.S., has not yet set an air date for the Christmas special. Fans are obviously hoping it will happen at or near the same time that U.K. viewers get to see it on BBC One in December.

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