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 uses his trademark deductive reasoning skills to find the killer of a college professor. Meanwhile, Watson gets a call from a former lover who needs her help in disproving hit-and-run charges.

Sherlock is called in to investigate when a supposedly uncrackable bank vault called “The Leviathan” is breached. Meanwhile, Joan is pleasantly surprised when Sherlock meets her family and defends her choice to become a sober companion.

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 a case involving a wealthy philanthropist who believes his incurable illness was given to him on purpose. Meanwhile, Sherlock tries to teach Joan his deductive skills by sending her to a suspicious dry cleaning establishment.

When a financial consulting company CEO is poisoned to death, a woman from Sherlock's past who emerges as a possible suspect.

When Sherlock’s brother Mycroft makes a surprise return to New York, his motivations cause discord between Sherlock and Joan. Meanwhile, a missing person case takes Sherlock and Joan inside the world of unmanned aeronautics.

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.