When Sherlock investigates what appears to be a series of murders at a top Wall Street firm, he may himself fall foul of a sociopathic slayer. Joan is set up on a blind date and shows investigative instincts.

Sherlock is reunited with “M.” a British criminal who appears to have followed Sherlock to New York. Meanwhile, Joan has reservations about leaving Sherlock and taking on a new client.

Sherlock pursues Martin Ennis, an unpredictable criminal, before he strikes again. During the investigation, Sherlock is forced to work alongside Kathryn Drummond, the FBI profiler who literally wrote the book on Ennis. Meanwhile, Watson faces eviction from her apartment.

When Sherlock's ex-drug dealer, Rhys, comes around for help, Watson is concerned about Sherlock's sobriety after Rhys implies that Sherlock on drugs made a better detective.

Sherlock investigates what he believes will be an open-and-shut case after Detective Bell is attacked. However, the case takes an unexpected turn when the suspected assailant is found dead. Meanwhile, Sherlock encourages Joan to learn self-defense.

When a beautiful young woman is murdered, Holmes and Watson investigate her ties to a billionaire CEO. Meanwhile, Mycroft makes a case for Sherlock to move back to London.

Joan discovers an unsolved murder connected to a rare prehistoric fossil when she reviews one of Sherlock's cold cases; Sherlock's responsibilities as a sobriety sponsor cross personal boundaries.

Business becomes personal for Sherlock when he is forced to partner with his estranged brother, Mycroft, to solve a case with life-or-death consequences.

When a serial bomber becomes active again after being dormant for six years, Watson seeks help from Shinwell Johnson, a former patient whose life she saved when she was a surgeon. Also, Holmes senses Watson is unhappy and questions whether she is still fulfilled by her detective career.

Holmes and Watson search for two murderers after a base jumper is shot out of the sky by one killer, and has his parachute sabotaged by another. Also, after Joan’s sister, Lin, thinks she sees Shinwell with a member of his old gang, Joan takes drastic measures to make sure he won’t be drawn back into a life of crime.

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.

Holmes’ former sobriety sponsor, Alfredo, asks for his help with an illegal endeavor in order to save his brother. Also, Holmes and Watson investigate the poisoning of a biology professor who was working on a secret genetics project prior to being killed.