'Sherlock' season 4 spoilers: EP hints 'darker' storyline; Sherlock to have major personality tweak

BBC One's "Sherlock" officially returns to the small screen in two days, and the network recently held a special screening of episode 1 in London. During a post-screening Q&A, it was revealed that fans will be seeing a different side of Sherlock Holmes (Benedict Cumberbatch) when season 4 kicks off.

The fourth season picks up with Sherlock back on British soil and Dr. John Watson (Martin Freeman) welcoming his first baby with his wife Mary (Amanda Abbington). According to Cumberbatch, his character will change in the sense that he becomes more "responsible for his actions." The "Doctor Strange" star went on to explain that Sherlock's personality tweak is a slow process that started when he first met John. He realized that the character "needed missing part of the jigsaw that is him," and it began a deep friendship that turned into a "humanizing element all the way through."

While much has been speculated about the effect of Watson's marriage and new baby, the latest addition to the family will not disrupt Watson's relationship with Sherlock in any way — similar to how it was in the original "Sherlock" story. However, showrunner Steven Moffat warns fans that they shouldn't expect the cheerful mood to last. "You can't live your life like that and have it work out forever," he said. "There will be consequences for that. You know, it's not a fairy tale. There will be a darker side for rest of the [season]," Moffat went on to say.

Titled "The Six Thatchers," the season 4 premiere centers on a mysterious case baffling the whole Scotland Yard. However, Sherlock is more interested in a seemingly unimportant detail. The mystery-solving duo tries to figure out why someone is destroying images of the late Prime Minister Margaret Thatcher, but they aren't sure if the culprit is just a madman on the loose or if a much darker purpose is at work. Also, something about Mary's past will be revealed.

The fourth season of "Sherlock" premieres on Sunday, Jan. 1, at 8 p.m. ET on BBC One.