拳手林健儿（吴奇隆 饰）不甘受黑社会大哥阿勇（陈松勇 饰）的摆布，拒打假拳，准备携家逃跑却被抓回……出身警察世家的杨重（金城武 饰）因行事莽撞杀死了线人，丢了公职又犯下杀人罪；一干天涯沦落人在火烧岛监狱服刑，股票大王吴世达（吴孟达 饰）在狱中成为众人觊觎的目标，幸有酒鬼阿梁（赵自强 饰）和林健儿出手相助，吴世达出钱令林健儿、阿梁和杨重在狱中得到照顾。不久，狱中贩毒集团大哥宫天保（楼学贤 饰）向当年抓获自己的杨重寻仇，杨重却以德报怨在阿勇袭击宫天保时出手阻挡。宫天保安排人手报复阿勇，自此与林健儿、阿梁等站到同一行列。典狱长（黄秋生 饰）欲从宫天保手中获得更大的利益，多次设计陷害宫等人，众人不堪忍受决定奋起反抗……A young police officer "makes a mess" of a police operation, shoots a man in self-defense, and ends up in prison. There he learns that the world isn't black and white, but has many shades of gray, and makes new friends and enemies.
