Sherlock assists Captain Gregson with a double homicide investigation that has the same MO as a series of murders from 13 years ago. That case led to the conviction of Paulson Crewes, who is still in jail, and gave Gregson his big break in the department. While Gregson tracks leads in an attempt to find a copycat killer, Sherlock starts to wonder if Crewes was guilty in the first place. Against Gregson's wishes, Sherlock digs into the old case in an attempt to learn what actually happened. After being frozen out by Sherlock, Watson pays a visit to his old rehab center in order to learn more about the enigmatic man in her care.