Sherlock tries to determine who has been killing weak and terminal patients at a hospital, making it look like they died of natural causes. Meanwhile, Sherlock learns more about Joan's decision to walk away from her surgical career.

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.