Sherlock's upcoming special will be set in Victorian London.
The show's co-creator Steven Moffat confirmed that the one-off 90-minute episode is entirely separate from other instalments.
Fans have speculated about the episode's setting after seeing stars Benedict Cumberbatch and Martin Freeman filming in 19th century clothing.
Moffat told Entertainment Weekly: "The special is its own thing. 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. [Co-creator Mark Gatiss] and me, we wanted to do this, but it had to be a special, it had to be a separate entity on its own.
"It's kind of in its own little bubble."
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The writer confirmed that filming for the special is almost complete, adding that he is very pleased with what they have produced.
Sherlock's special will air later this year ahead of a new three-episode series.
Watch Moriarty star Andrew Scott discuss the success of Sherlock with Digital Spy below:
