Sherlock is called to London to revisit an old case, where he is forced to face his older brother, Mycroft. Although the siblings suffered a drastic falling-out a few years earlier, Mycroft allows Sherlock and Joan to stay in his new home, 221B Baker Street. With Joan in the middle, the brothers are forced to confront their very complicated history.

When Captain Gregson’s home is invaded, Holmes and Watson work to find the masked gunman before he strikes again. Meanwhile, Gregson gives his team some insight into his private life.

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 investigate a clown’s murder upstate, they discover a connection between his death and the creation of a new weaponized virus. Also, Watson helps Bell when he’s targeted by his new love interest Chantal’s ex-husband in attempt to sabotage her career at the district attorney’s office.

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.