In a recent trailer for the upcoming fourth season of the popular BBC detective TV series "Sherlock," the titular character portrayed by Benedict Cumberbatch and his acquaintance John Watson (Martin Freeman) can be seen in ankle-deep water at their flat at 221B Baker Street.

Like the show itself, the trailer is shrouded with a lot of mystery. It is a brief clip of the flooded apartment of the pair. Both of the detectives have stern expressions on their face, like they are thinking well and hard about a particular case. The duo is in a cluttered room, which is believed to be clues for the upcoming season.

Although the teaser for the fourth season isn't straightforward, fans of the show have come up with several theories based on what was given. A few of the popular theories that fans have deduced suggest that Jim Moriarty (Andrew Scott) will be back to haunt the titular character and that Tom Hiddleston will be portraying the third Holmes brother.

The Independent suspects that the tea cup that can be seen on the floor of the apartment was the one Moriarty drank from before when he paid the detective a visit. Since Moriarty is assumed to be dead, he might find ways to haunt the titular character in the episodes to come.

Furthermore, the news outlet believes that Hiddleston will be portraying a third Holmes brother, which they once referred to as "the other one," and who they call Sherrinford. In the latest teaser, a red leather book can be seen amid the clutter, which is believed to be a copy of Shakespeare's Henry V. Hiddleston. The publication sees this as a clue for the actor's alleged role.

Aside from the detective duo, the rest of the regular cast members — Rupert Graves as Greg Lestrade, Una Stubbs as Mrs. Hudson, Mark Gatiss as Mycroft Holmes, Amanda Abbington as Mary Morstan, and more — are expected to reprise their roles for the upcoming film. This season, Toby Jones joins the cast as the villain Culverton Smith. He will appear in the second episode of the show titled "The Lying Detective," which is based on Sir Arthur Conan Doyle's "The Adventure of the Dying Detective."

The fourth season of "Sherlock" is expected to return to BBC on New Year's Day, which is three years after they launched the third season.