A trailer for a Christmas special for the BBC series Sherlock has been released and there's a twist in the tail for the one-off episode: it's set in the 19th century.

The trailer shows Benedict Cumberbatch's Sherlock and Martin Freeman's Watson solving mysteries in the same time period as Sir Arthur Conan Doyle's original works.

The pair race around the cobbled streets of London, illuminated by lantern or candlelight and everyone look appropriately old-timey with their top hats and elaborate mustaches.

None of the sly wit the show is known for is lost with the change of setting, however, with the teaser closing with Watson irritably insisting Sherlock wear his trademark deerstalker as they leave 221B Baker St.

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"You're Sherlock Holmes," he says. "Wear the damn hat."

For Sherlock fans hanging out for season four of the British drama, this will be a welcome treat from creators Steven Moffat and Mark Gatiss.

Each series of the much-loved show is only three episodes long and the wait time in between each series has been more than a year.

Season three wrapped up in January last year.

- nzherald.co.nz