Sherlock fans will have the chance to put their own powers of deduction to the test with a special case written to be solved on social media.
Steven Moffat and Mark Gatiss, the show's writers, will be tweeting live to fans in character as Sherlock to help crack a brand new mystery.
The case, dubbed #SherlockLive and penned by Joe Lidster, will unfold over 30 minutes on the BBC One Twitter account, @BBCOne, and will be available to follow as a liveblog on bbc.co.uk/sherlock from 8pm on Tuesday.
The show, starring Benedict Cumberbatch and Martin Freeman, returned to television on January 1.
Mark, who also plays Mycroft in the series, recently made a poetic response to critics who attacked the new episodes for being too much like James Bond.
The star, 50, wrote a five-verse piece, complete with rhyme, explaining how Sir Arthur Conan Doyle's famous detective had always been a skilled fighter.