Benedict Cumberbatch and Martin Freeman as Sherlock and Watson
Benedict Cumberbatch and Martin Freeman as Sherlock and WatsonFB / Sherlock

A cryptic tweet by "Sherlock" co-creator Mark Gatiss has indicated that filming has commenced for Season 4 of the detective drama.

"I spy Series Four #Sherlock," Gatiss wrote alongside a picture of a skull that is familiar to "Sherlock" fans.

Plot:
The plot of Season 4, starring Benedict Cumberbatch as Sherlock Holmes and Martin Freeman as John Watson, is under wraps, but the show-runners revealed in an earlier interview that it will be about consequences. Season 4 is also said to be tougher and darker than the previous instalments.

"Chickens come home to roost. It's dark in some ways - obviously it's great fun and a Sherlock Holmes romp and all that - but there's a sense of things coming back to bite you," co-creator Steven Moffat said in an interview with Entertainment Weekly. "It's not a safe, sensible way to live. It's hilarious and exhilarating some days, but some days it's going to be bloody frightening."

Also, rumours doing the rounds indicate that the much-loved Mary Watson, played by Amanda Abbington, will be killed off in Season 4.

Air Date:
Like the previous seasons, the fourth instalment of the BBC detective drama is set to air in early 2017. There will be three 90-minute episodes in Season 4.

Guest stars:
Besides Cumberbatch, Freeman and Abbington, Season 4 will see Una Stubbs, Louise Brealey, Rupert Graves and Jonathan Aris returning to reprise their roles. If rumours doing the rounds hold any truth, then Tom Hiddleston will essay the role of the third Sherlock brother.

However, the creators are yet to address if Hiddleston is a part of the series.