Holmes and Kitty investigate the murder of a postal store owner whose death is connected to the illegal diamond trade. Meanwhile, Gregson's career is placed in jeopardy when he hits a fellow NYPD officer.

Holmes and Watson investigate the murder of a professional skeptic, a man who debunked paranormal, religious and scientific phenomenon for a living. The case becomes urgent when his homicide reveals a potential threat to homeland security. Also, Captain Gregson’s daughter and fellow NYPD officer, Hannah, asks Watson for help with one of her cases.

Holmes and Watson enter the world of nuclear security when they investigate the murder of an ex-Yakuza gang member. Also, Captain Gregson is blindsided by a stunning personal confession from his daughter, Hannah, a police sergeant.

Holmes and Watson find themselves on the hunt for a serial killer after a murder rocks their inner circle. Also, Holmes’ neurological recovery regresses and his sobriety wavers as his commitment to catching the killer grows into an obsession.

Holmes and Watson face their greatest challenge to date when a murder investigation threatens to destroy their partnership.