這部電影不僅在香港創下極高票房，更為香港長期以來就很受歡迎的警匪類型電影帶上更高顛峰。Tony飾演臥底警察陳永仁，從年輕時就混入幫派老大身邊做臥底，他一直期望有一天可以完成任務，重回警察崗位。Tony的角色內斂壓抑，演來絲絲入扣，不做第二人選。他也因本片再度連獲金馬獎、香港金像獎的多項影帝寶座。

Chan is a cop who's been assigned to undercover work inside the Triads for so long that he's been able to rise through the ranks to a position of some authority. Lau, meanwhile, is a secret member of the Triads who has infiltrated the police force with an equal level of success. As they feed their bosses information on the plans and counter-plans of the organizations they pretend to serve, they both begin to feel the stresses of their double lives as they become torn between the oppressive obligations they owe to their superiors and the growing camaraderie they share with the foot soldiers around them. As the two organizations become increasingly aware of the moles in their midsts, the race is on for Chan and Lau to try and get out of the game alive.