Sherlock assists with a double homicide that has the same M.O. as a series of murders that took place 13 years ago. Meanwhile, after being frozen out by Sherlock, Watson visits his old rehab center in order to learn more about the mysterious man in her care.

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.

Holmes and Watson uncover the identity of a serial killer whose case has gone cold and find themselves butting heads with the NYPD.