Holmes' former protégé, Kitty Winter, returns to New York after a three-year absence to warn him that a killer is eliminating everyone involved with a case they worked on together in London, and they're the next targets. As Sherlock, Kitty, Joan and the NYPD join forces to find the perpetrator, Watson discovers Kitty has a life-changing secret.

The police call in Holmes and Watson to consult on a homicide "heard" by a cutting edge gunshot detection system when they can't find any tangible evidence that a crime was committed. Also, Sherlock and Joan become suspicious of Shinwell when he's the target of a drive-by shooting that reveals a connection between him and an unsolved murder.