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.

Holmes and Watson come face to face with Moriarty when their one-time adversary is brought in as a consultant on a kidnapping case. Meanwhile, Detective Bell continues his struggle on the road to recovery.