"Sherlock" Season 4 launched with the episode titled"The Six Thatchers," which showcased an interesting mystery, a lot of action and funny quips. The episode also revealed a twist for one of the main characters.

At the start of "Sherlock," Mycroft (Mark Gatiss) informed his government cronies that Sherlock's (Benedict Cumberbatch) involvement in Magnussen's (Lars Mikkelsen) death should not be known to anyone. He continued to show an altered surveillance footage that would exonerate his brother, which keeps Sherlock safe from the lingering issue in "His Last Vow." However, Sherlock is still waiting for Moriarty to execute his next move. He said that he is the target and they can only try to guess where the villain will attack next.

In the meantime, Sherlock decided to return to crime-solving. However, Mary (Amanda Abbington) went into labor and John (Martin Freeman) rushed her to the hospital. She eventually delivered a baby girl, Rosamund Mary, and asked Sherlock to be her godfather.

Meanwhile, Cabinet Minister David Wellsbury received a call from his son Charlie at his 50th birthday party. One week later, a drunk driver smashed into David's driveway and exploded, with Charlie in it. While trying to solve the case, Sherlock noticed that there were six plaster busts of Margaret Thatcher in various memorabilia sets were missing, except for one. The clue led him to the burglar.

Later on, it was revealed that Mary was part of an elite group of mercenaries that did missions for the highest bidder. They kept all their secrets in memory sticks. A.J. was allegedly trying to have her killed, although it turned out that Mrs. Norberry was the real target. When Sherlock and the police tracked Mrs. Norberry, she fired her gun at Sherlock but Mary dove and took the bullet. Mary died, saying that being John's wife was the only life worth living.

The final scene in "Sherlock" showed Mrs. Norberry being taken away by the police. John walked quietly through a graveyard, while Sherlock met with a therapist. Afterwards, Sherlock received a CD in the mail. It was a pre-recorded video from Mary, to be played after her death. She gave him one final case to save John Watson, although the latter is still grieving and refuses to see him.

The next episode for "Sherlock" season 4 airs on the BBC on January 8. It is titled "The Lying Detective."