20世纪60年代，中国香港鱼龙混杂，混乱不堪。英国人治下的皇家警察贪婪无度，甚至与黑道相互勾结，搜刮民脂。年轻探长雷洛（刘德华 饰）踌躇满志，小心周旋在英国人和华人上司之间，并瞅准一切机会攀登权力山峰。在某次火并事件中，雷洛看中了敢打敢杀的伍世豪（甄子丹 饰）。来自南洋的伍世豪义字当头，情愿为兄弟两肋插刀。他凭借一股闯劲儿在黑道打出自己的位置，同时借着与雷洛相互帮衬的势头而声名鹊起。最终，刀头舔血的雷洛和伍世豪共同成为分别执掌白道和黑道的重量级人物。 　　而在这一过程中，这对兄弟也渐渐发生分歧。当风暴来临之际，一切即将变化……The year is 1963, an illegal immigrant named Ho (Donnie Yen) sneaks into British-ruled Hong Kong. Equipped with guts and combat skills, he plunges into the underground world getting into constant conflict with rivals. After many adversaries, the once good-natured man is physically crippled and turns into a monster more atrocious than all the most corrupted cops and ruthless drug dealers. Ho eventually emerges as the most powerful drug lord under the control of Chief Detective Sergeant, Lee Rock (Andy Lau). However, with the establishment of Independent Commission Against Corruption (ICAC) by the law enforcement in 1974, Rock is forced into premature retirement. But Ho still insists he would not be stopped as he determines to become the sole dictator of the drug empire.