There will be more mysteries to solve for the sleuth of Baker Street.

The BBC confirmed Saturday that filming on a fourth series of the hit detective drama “Sherlock” will start  next spring. The modern-day update of Arthur Conan Doyle’s classic detective stories stars Benedict Cumberbatch as Sherlock Holmes and Martin Freeman as his stalwart companion, Dr. Watson.

Since “Sherlock” first aired in 2010, Cumberbatch has become a global star, Academy Award-nominated for “The Imitation Game.” Freeman has starred in “The Hobbit” movie saga and in TV’s “Fargo.”

The BBC in Britain and PBS’ “Masterpiece” in the United States have also announced they will broadcast a 90-minute “Sherlock” special episode on Jan. 1. Titled “The Abominable Bride,” it sends the characters back to the stories’ original late 19th-century setting.

“We want to see them do it in the proper [period] outfits, just once,” said showrunner Steven Moffat.

The trailer for the Victorian-era holiday episode is available on YouTube and elsewhere online. It shows Freeman’s Watson sporting a bushy mustache.

It’s been a long wait for fans of the show, which had its third season debut in January 2014.

Cumberbatch has been starring in a London production of William Shakespeare’s “Hamlet.” A recent Los Angeles Times review said his portrayal “combines classical clarity with an emotional openness.”