The first pictures of the duo on the BBC One show saw them in Victorian garb and now co-creator Steven Moffat has confirmed that it will be set in that era.
Moffat told Entertainment Weekly that the special is independent from the show's highly anticipated fourth season, which will likely appear in early 2016.
"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," he said.