When a counterterrorism raid goes tragically wrong, Detective Sergeant Steve Arnott refuses to participate in a cover-up. Ostracised by colleagues, he transfers to an anticorruption unit, AC-12, led by Superintendent Ted Hastings. Arnott finds his target is the city's top detective, Tony Gates. Can Gates really be as good as he appears? Arnott must engage in a cat-and-mouse struggle to uncover Gates' secret.