After a mistaken shooting kills a man during a counter-terrorist operation, Detective Sergeant Steve Arnott is transferred to the police anti-corruption unit led by Superintendent Ted Hastings, and alongside Detective Constable Kate Fleming.
When an attack on a police convoy leaves one surviving officer behind, she becomes the prime suspect of the investigation. When the team's attempt to get evidence on her ends in tragedy, they become determined to see justice served, but the case turns out to be more complicated than they thought.
With suspicions surrounding the actions of Sergeant Danny Waldron's squad, AC-12 takes charge of the investigation, but a sudden death derails their efforts and sends them into another direction.
The newest case assigned to the AC-12 is the investigation of D.C.I. Roseanne Huntley who is suspected of overlooking evidence in a murder case just so that she could make an arrest.
With an attack launched by an organized crime group, the AC-12 looks into the possible corruption of undercover officer John Corbett who might have aided them.