缉毒总督察李振邦（张家辉 饰）和卧底警员江铭（陈伟霆 饰）秘密执行追捕毒贩计划，无奈遭遇险境，江铭被迫游走黑白边缘，逐渐失控。为坚守底线，昔日队友互生杀意。猜忌、愤怒、疯狂、失望……全员丧失理智，濒临崩溃边缘！

Hong Kong Customs and Excise Department officers Chow Ching-lai and Cheung Wan-nam share a close mentor-student relationship. On a case involving seized smuggled arms, Ching-lai is sent to take part in a joint operation with Interpol to capture a reclusive arms dealer. However, Wan-nam makes a drastic decision that further complicates the case.