The new trailer for "Sherlock" season 4 suggests that the upcoming season will be a gloomy one.

The first episode of the upcoming series is titled "The Six Thatchers," which is reportedly in reference to a former prime minister of the United Kingdom. On the other hand, the second episode, titled "The Lying Detective," is an adaptation of Arthur Conan Doyle's "The Dying Detective."

The one-minute trailer was released on YouTube this week and teases that season 4 will see more dangers and obstacles for both Watson and Holmes. It also features action-packed scenes from the upcoming season.

In one of the scenes, a voice says, "Something is coming. Perhaps it's Moriarty. Possibly it's most certainly not," affirming that another enemy is coming to the show next season. Fans speculate that the said conversation has something to do with the danger looming Culverton Smith's way. There are speculations that Smith was the one behind the murder of Holmes' nephew in the previous season of "Sherlock."

The trailer also shows Holmes telling everyone about his rare and lethal illness.

Since "Sherlock" season 4 will mark the end of the series, fans expect that it has a great deal of action and drama in store for viewers. Talking about the end of the series in a recent interview, Benedict Cumberbatch said that it feels like "the end of a time" for him.

"Sherlock" season 4 is set to air on BBC One in the United Kingdom and PBS in the United States on January 1.