The true story of Whitey Bulger, the brother of a state senator and the most infamous violent criminal in the history of South Boston, who became an FBI informant to take down a Mafia family invading his turf.

Chan Wing Yan, a young police officer, has been sent undercover as a mole in the local mafia. Lau Kin Ming, a young mafia member, infiltrates the police force. Years later, their older counterparts, Chen Wing Yan and Inspector Lau Kin Ming, respectively, race against time to expose the mole within their midst.