Sherlock and Joan hunt for the accomplice of a murdered blackmailer who targeted families of rape victims. Meanwhile, Sherlock resists celebrating his first year of sobriety.

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.

A serial killer known for biting his victims appears to be killing again, even though he was presumed dead.

When Sherlock and Joan join forces on a double homicide, Sherlock’s new apprentice threatens the investigation when she allows her jealousy of Sherlock and Joan's work rapport to override her better judgment.

When Watson joins forces with Holmes and Kitty to locate a missing nurse she used to work with, the woman’s trail leads them to another person who has recently disappeared.

Holmes’ tentative reconciliation with his father suffers a setback when Holmes and Watson discover Morland has been keeping a secret that threatens all of their safety. Also, Holmes and Watson investigate two murders connected to a for-profit college and its illegal recruitment practices.

When Holmes’ investigation into the attempt on Morland’s life pushes their strained relationship to the breaking point, the identity of Sherlock’s mother is revealed. Also, when three gang members are murdered, Holmes and Watson are amazed when an elderly woman emerges as their prime suspect.