As Watson and Holmes start a new chapter in their lives with their work with Scotland Yard, Bell tries to get Gregson to reconcile with them. Watson and Holmes receive bad news from back home.
As Watson and Holmes look into the attempt on Gregson's life, they have to keep a low profile due to Holmes' problems with the law, which further complicates their mission.
As Watson joins Bell on an investigation into a murder related to customs evasion, Holmes is forced to enlist his father's help to facilitate his return to the US.
With Gregson's shooting bearing the evidence of a larger conspiracy, Holmes and Watson balance their investigation with their recent case, which involves an explosion born out of tampering with traffic lights.
As Holmes and Watson try to figure out the link between a wave of seemingly unrelated murders, they get hired by a billionaire to identify the person who has been sending him threats.
As Odin Reichenbach makes Watson and Holmes an offer that might enable them to prevent more crimes from happening, Bell enlists their help to find a rogue officer before he crosses the line and causes a tragedy.
As Gregson returns to work, he looks into the reason one of his best detectives has tendered his resignation. Watson and Holmes are assigned to a case where the murder victim made a habit of robbing other criminals.
When Cassie Lenue contacts Holmes and Watson to ask for help, they come to suspect that she has something up her sleeve that she is not sharing with them.
With the possibility of the return of an inactive serial killer, the investigation is made more complicated as tension rises between Watson and Holmes due to the former's distrust of Holmes's actions with Odin Reichenbach.
As Odin Reichenbach puts his crime prevention program to the test, Watson and Holmes hope to get ahead of him as his program identifies a potential criminal.
As Odin Reichenbach's crime prevention program goes public, Holmes is forced to put his differences with his father aside to find a way to halt the program.
With Holmes and Watson onto him, Odin Reichenbach unknowingly provides them with a way they could use to bring him down, but it's up to them to figure out how.
As things come to a head with Odin Reichenbach, Holmes receives unexpected info about Jamie Moriarty, just as he finds out unsettling news about Watson.