波士顿南部的警局里正在紧锣密鼓的准备一次围剿行动，警方准备一举除掉爱尔兰黑帮。科林（马特·达蒙饰）和比利（莱昂纳多·迪卡普里奥饰）都是警校刚毕业的优秀学生，因为比利曾经生活在南部最底层的街区，比利没有上岗就被派去黑帮当卧底。科林因为缉私有功被领导派去对付黑帮老大弗兰克·卡斯特罗（杰克·尼科尔森饰）的小组，多次立功成为警局不可或缺的人才。实际上科林14岁时就被弗兰克培养长大，在警局当眼线，每次都会提前把计划告诉弗兰克。

比利多次跟着弗兰克出生入死逐渐成为得力手下，但是双方多次行动失败，让黑帮和警方都意识到内部出现了间隙，比利和科林也因为双重身份十分困扰。随着冲突逐渐明朗，比利和科林不断受到身份暴露的风险。

The police station in south Boston is preparing for a siege operation, and the police are preparing to get rid of the Irish gangsters in one fell swoop. Colin (Matt Damondecoration)andBilly( Leonardo DiCapriodecoration) are all police schools An outstanding student who just graduated, because Billy used to live in the lowest neighborhood in the south, Billy was sent to the gangsters as an undercover agent before he was on the job. Colin was sent by the leader to deal with the boss of the gangster because of his crimes against private individualsFrank Castro (Jack Nicholsondecoration)'s team, has made many meritorious deeds and has become an indispensable lack of talent. In fact, Colin was brought up by Frank when he was 14 years old. He worked as an eyeliner in the police station and told Frank the plan in advance every time.

Billy followed Frank through life and death many times and gradually became an effective man, but the two sides failed many times, which made the gang and the police realize that there was a gap inside, and Billy and Colin were also troubled by their dual identities. As the conflict becomes clear, Billy and Colin are constantly at risk of having their identities exposed.