After refusing to help cover up an unlawful and fatal shooting by members of his team, Detective Sergeant Steve Arnott is transferred to Anti-Corruption Unit 12, AC-12.Run by zealous Superintendent Ted Hastings, the department is responsible for investigating possible corruption within the Central Police force. Working alongside DS Arnott is Detective Inspector Kate Fleming, who is an undercover specialist. With the help and oversight of Hastings, Arnott and Fleming take on difficult cases, which lead from "laddering" to mishandling evidence.