从小一起长大的好兄弟马昊天(刘青云 饰)、张子伟(张家辉 饰)和 苏建秋(古天乐 饰)共同效力于警队扫毒科。在一次临时改变计划的行动后,建秋因为卧底身份不能与妻子(袁泉 饰)过正常生活而心生退意,但在阿天与子伟的劝说下,三人决定进行最后一搏。建秋跟着毒贩老大黑柴(林国斌 饰)前往泰国与毒贩Bobby(卢惠光 饰)进行对接,目的是见到行动的最大目标“八面佛”(卢海鹏 饰),阿天则和子伟以及同事阿益(吴廷烨 饰)进行跟进。由于泰国警方的配合不力导致建秋身份暴露,虽然建秋答应阿天继续把交易完成,但失败的导火索已经埋下。交易当天警方几乎全军覆没,危急之时阿天挟持了八面佛的女儿缅娜(宝儿 饰)准备逃命,八面佛承诺只要还回缅娜阿天可以带走建秋和子伟中的一个,没被选中的则要被推进鳄鱼池,面对抉择阿天及进退两难……Three lifelong friends working in the Hong Kong Police Department's Narcotics Bureau get caught up in a case with Southeast Asia's most powerful drug lord, but when the sting operation fails, they are forced to make a devastating decision - two can live, but one must die. Five years later, the two vow revenge for their fallen friend, but when they end up competing against each other for their own lives, will brotherhood be sacrificed again?