Sherlock Season 4 plot spoilers: BBC drops more clues; here's what we know

Season 4 of Sherlock will kick off on New Year's Day with The Six Thatchers, the first of the three new episodes, and a new promo released on Wednesday hints at disaster striking Benedict Cumberbatch's Holmes and Martin Freeman's Watson. The new video shows the sleuthing duo looking pensive as their Baker Street living room gets flooded.

The first episode of the season is said to be based on Sir Arthur Conan Doyle's The Adventure of the Six Napoleons, which saw Sherlock being presented with a case of smashed busts of Napoleon that were left abandoned in various locations. The television adaptation sees a mysterious stranger smashing up images of the late Margaret Thatcher.

"One mysterious case in particular baffles Scotland Yard – but Sherlock is more interested in a seemingly trivial detail," a statement released by BBC noted. "Why is someone destroying images of the late Prime Minister Margaret Thatcher? Is there a madman on the loose? Or is there a much darker purpose at work? Something with its roots deep in Mary Watson's past…"

The statement released by BBC also noted that the new episode will see Sherlock waiting to see where Moriarty will make his posthumous move. The Christmas special that aired last year saw Sherlock waking up from a drug-induced sleep, saying Moriarty is dead. At the same time, Sherlock seems to have figured out what Moriarty is going to do next.

Season 4 of Sherlock will see Watson and Mary becoming parents and latest images released last week showed Sherlock looking rather quizzically at Watson's new baby.

The villain of Season 4 is Toby Jones' Culverton Jones, a character from Arthur Conan Doyle's short story The Adventure of the Dying Detective.