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.

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

Tensions rise when Holmes and Watson are forced to partner with Gareth Lestrade, Sherlock’s former Scotland Yard colleague, on a bombing investigation.

Holmes and Watson come into contact with MI-6, the British intelligence agency, when a standard murder investigation turns out to have international and political implications. Meanwhile, in the wake of a crisis, Watson reevaluates her relationships with Sherlock and his brother, Mycroft.

Sherlock takes on the case of a stolen artificial intelligence software program, but seeks out Joan when he'd rather disprove the computer's abilities than find the thief. Joan meets Sherlock head-on when she finds out he's met with Andrew without telling her.

Watson begins her new job as an in-house investigator for an insurance firm. However, her attention shifts to a case that is personal for both her and Holmes when they join forces to eliminate an imminent threat to Kitty.

When Holmes is implicated in the murder of a woman who was killed during the height of his addiction, he investigates himself to find their connection. As Holmes retraces his steps with Watson’s help, the clues make his confidence in his own character waver.

Holmes and Watson look into the death of a member of Holmes’ online beekeeping community who was a researcher looking into a deadly Northeast honey bee outbreak. During the investigation, they meet Tara Parker, a brilliant entomologist, academic and the leader of New York’s beekeeping society. Also, Captain Gregson enlists Watson’s services in an off-book investigation that helps him make a life-changing decision.

When Watson’s stepfather writes a crime novel based on her work with Holmes without her knowledge, an underlying issue in their relationship is revealed. Also, as Holmes and Watson investigate a retired FBI agent’s murder, they discover the man’s death is connected to an unsolved case from the victim’s career.

Holmes and Watson search for a computer hacker responsible for three murders, including the death of a Russian billionaire who was visiting New York City. Also, Joan is surprised to receive a solo dinner invitation from Sherlock’s father.

Sherlock and Joan search for a murderer who used poisonous mushrooms to kill a botany professor and a group of the instructor’s students. Also, when Joan is disturbed by loud noise next door, she discovers her neighbor, Trent, is renting out his home through a vacation site that caters to hard partiers.

When Holmes and Watson discover the perpetrator behind the attempt made on Morland’s life, they find a shocking connection to themselves.

When the leader of a local street gang abducts Holmes to compel him to find the person responsible for a hit on his men, Holmes and Watson get caught in the middle of an international hunt to recover a centuries old, priceless artifact. Also, Watson debates whether a personal favor Shinwell asks of her is outside her comfort zone.

Holmes and Watson have plenty of suspects when they investigate the murder of a man who led a secret life as a vigilante, “catfishing” sexual predators on dating sites and publicly shaming them. Also, when Shinwell’s criminal record prevents him from finding a full-time job, Holmes disagrees with Watson’s plan to help him.

A man takes hostages, including Joan, and threatens to execute them unless Holmes finds the person responsible for the death of his son. As Sherlock investigates, the captor gives him 16 hours to find the perpetrator before the statute of limitations on the crime runs out.

When Detective Bell’s girlfriend, Chantal, is assaulted and her ex-husband is killed soon after, in what Holmes and Watson discover is a staged suicide, they search for one suspect behind the linked crimes. Also, as Bell fights the urge to seek his own brand of justice for Chantal, Sherlock makes a heartbreaking realization about the detective’s past.

Watson and her half sister, Lin, have conflicting reactions when their estranged biological father dies. Also, Holmes and Watson find themselves on the hunt for a stolen plutonium shipment they fear will be used to make a dirty bomb after a department of energy inspector is killed with a lightning gun.

Watson considers a major life change after reading her own therapy file while she and Holmes investigate the murder of her former psychologist.

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.

Holmes engages an old foe to protect his father after he learns Morland is in danger of being killed. Also, Holmes and Watson search for a killer connected to the tobacco industry when a lawyer who was in the process of reviewing a major cigarette distributor’s finances is murdered.

Holmes and Watson race to help Detective Bell when he becomes a victim of a bio-terrorism attack at the precinct and the station is quarantined. During the lockdown, Captain Gregson and Detective Bell begin to suspect they are trapped inside with the perpetrator. Also, Watson faces the difficult task of helping her mother, Mary, accept that her advancing Alzheimer’s disease requires her to receive more medical care.