As Sherlock reels at the reappearance of his former lover, Irene Adler, a series of flashbacks unravel the tumultuous events that led to his downfall into addiction. Meanwhile, he and Joan once again find themselves in the crosshairs of the enigmatic Moriarty.

Sherlock is called to London to revisit an old case, where he is forced to face his older brother, Mycroft. Although the siblings suffered a drastic falling-out a few years earlier, Mycroft allows Sherlock and Joan to stay in his new home, 221B Baker Street. With Joan in the middle, the brothers are forced to confront their very complicated history.

Rising tension between Sherlock and Joan brings their partnership to a crossroads, but they endeavor to put their differences aside while they help Sherlock's brother, Mycroft, who faces accusations of treason and murder.

When a debt collector is murdered, Holmes and Watson try to narrow down the suspect pool from thousands of consumers who owed the victim money. Meanwhile, Watson’s relationship with Andrew progresses when he asks her to meet his father for the first time.