Holmes and Watson face their greatest challenge to date when a murder investigation threatens to destroy their partnership.

As Kitty begins to unravel due to the imminent threat to her safety, Sherlock and Joan intensify their efforts to help her. Also, the origin of Sherlock and Kitty’s relationship is revealed.

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.

As Sherlock reels at the reappearance of his former lover, Irene Adler, a series of flashbacks unravel the tumultuous events that led to his downfall into addiction. Meanwhile, he and Joan once again find themselves in the crosshairs of the enigmatic Moriarty.

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.

As Sherlock reels at the reappearance of his former lover, Irene Adler, a series of flashbacks unravel the tumultuous events that led to his downfall into addiction. Meanwhile, he and Joan once again find themselves in the crosshairs of the enigmatic Moriarty.

Sherlock agrees to take a case from a man claiming to be Moriarty, hoping it will bring him closer to finding his nemesis. Meanwhile, Gregson encourages Joan to give up the dangerous line of work she has chosen and go back to being a sober companion.

Holmes and Watson come face to face with Moriarty when their one-time adversary is brought in as a consultant on a kidnapping case. Meanwhile, Detective Bell continues his struggle on the road to recovery.

Tech billionaire Odin Reichenbach inadvertently provides Holmes and Watson with a lead that could give them evidence that brings him to justice.

Amidst Holmes and Watson’s battle with tech billionaire Odin Reichenbach, the investigators receive word of their old enemy and Sherlock’s former love, Jamie Moriarty.

Sherlock resumes his hunt for Moriarty when Sebastian Moran reveals that a man who was thought to be dead from a heart attack was actually on the list of intended victims from Moriarty.

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.

Holmes and Watson find themselves on the hunt for a serial killer after a murder rocks their inner circle. Also, Holmes’ neurological recovery regresses and his sobriety wavers as his commitment to catching the killer grows into an obsession.

Holmes and Watson join forces with Holmes’ father, Morland, to enlist his vast criminal network to help disassemble tech billionaire Odin Reichenbach’s crime prevention system.

Sherlock investigates what he believes will be an open-and-shut case after Detective Bell is attacked. However, the case takes an unexpected turn when the suspected assailant is found dead. Meanwhile, Sherlock encourages Joan to learn self-defense.

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

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.

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

Holmes and Watson’s deadly conflict with Sherlock’s former friend Michael intensifies when their newest victim looks to be the serial killer’s latest prey.

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.

Rising tension between Sherlock and Joan brings their partnership to a crossroads, but they endeavor to put their differences aside while they help Sherlock's brother, Mycroft, who faces accusations of treason and murder.

Holmes and Watson take on an investigation of a personal nature when Holmes’ recovery sponsor, Alfredo, suddenly disappears. As they delve into Alfredo’s life for clues, Holmes questions whether their friend has been abducted, or worse, if he suffered a relapse into addiction.

Captain Gregson’s colleague from Narcotics asks Holmes to orchestrate the perfect heist inside the heavily armed and virtually impenetrable compound of a drug-dealing biker gang. Also, when Watson assists Fiona Helbron, who helped her on a recent case, she makes a surprising discovery about Holmes.

Holmes and Watson’s lives are threatened by an influential, worldwide criminal network connected to the murder attempt on Morland Holmes. Also, Sherlock’s grievances with his father threaten their alliance to bring their mutual adversary to justice.

Holmes’ former protégé, Kitty Winter, returns to New York after a three-year absence to warn him that a killer is eliminating everyone involved with a case they worked on together in London, and they’re the next targets. As Sherlock, Kitty, Joan and the NYPD join forces to find the perpetrator, Watson discovers Kitty has a life-changing secret.