The hit BBC drama "Sherlock" returns on Jan. 19. BBC

“His Last Vows,” the final episode of the third season of “Sherlock,” is a thrilling 90-minute ride of laughter, terror and surprises, in a return to form for the series, which had taken a comedic turn in the previous two episodes.

For those who haven't watched the episode yet, be warned - there might be spoilers ahead. The third episode of the third season, which was also its last, makes a shocking revelation that Mary Morstan, wife of Dr. John Watson and the new member of Sherlock Holmes' team, is an assassin.

Mary’s arrival at Charles Augustus Magnussen’s office, the “arch-villain” of the episode, reveals that she is not a nurse but a secret agent-turned-assassin. And, it's shown that she comes to the office to shoot Magnussen, whom Sherlock is planning to take down.

Sherlock’s brilliant plan, which involves trading Mycroft’s information-laden laptop for Magnussen’s file to gain access to the vault, has only one problem. And that is, Magnussen does not have a vault and instead stores all the information in his mind palace.

Sherlock, who is being framed for selling government secrets, discovers that the mastermind Magnussen has a mind palace, and to neutralize his threat, Sherlock shoots Magnussen in front of Mycroft and the cops.

The major twist in the final episode comes at the end when Sherlock gets ready to go on another dangerous mission. As Sherlock and Dr. Watson bid goodbye and the detective boards the plane, Jim Moriarty’s face appears, saying “Did you miss me?” repeatedly.

Mycroft calls Sherlock and tells him that he needs to return, though the plane hasn’t even been gone more than four minutes.

According to reviews of the season finale, the episode is reportedly said to be the best one of the season. The fourth and fifth season of "Sherlock" has been announced by creator Steven Moffat but production dates are yet to be mentioned.