The BBC shared a photo of the show's stars, Benedict Cumberbatch and Martin Freeman, dressed in period costume for the upcoming episode expected to air around Christmas.
Not much is known about the upcoming special, but it's reportedly set in 1895, and will perhaps be quite a change from the fast-paced setting used for previous Sherlock series.
The photo comes ahead of the Sherlock panel today at Comic-Con in San Diego. Co-creator Steven Moffat, producer Sue Vertue and actor Rupert Graves, who plays Sherlock's colleague DI Lestrade, will discuss the show.
Series three of Sherlock ended last year but the follow-up fourth series is due to return to screens in 2017.
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