20世纪50年代末60年代初的香港，警匪勾搭，乌烟瘴气。其中，九龙城寨更是一个龙蛇盘踞，令人闻风丧胆的混乱所在。无所事事的小青年阿豪（刘青云 饰）和阿乐（吴镇宇 饰）偶然混迹于此，为这里的乱世气氛所感染，乐不思蜀。他们合伙赌马诈钱，惹来庄家追杀，幸妥事先城寨大哥肥宝出手方保住性命，二人的命运也自此发作转变。阿豪拜入肥宝门下，后与肥宝的小姨子大胆英（彭玲 饰）相恋，他果敢勇猛，风头无两；阿乐当上警察，与曾和阿豪有染的女孩菲菲（朱茵 饰）结婚。为了让虚荣的妻子过上幸福生活，他不惜选择贪污蜕化。两个分处黑色道的好兄弟，携手打拼出一个属于他们自己的天下…… At the end of 1950s and early 60s Hongkong police collusion, The atmosphere was foul. Among them, the Kowloon Walled City is a snake entrenched, feared the chaos. Idle young Arab (Lau Ching WAN) and ale (Francis ng ornaments) accidental blend in this, for the chaotic atmosphere here the infection, reluctant to leave. They form a partnership to bet horse cheat money, attracted the dealer to kill, is fortunate enough to was walled city brother Po hand side to save their lives, the fate of the two has since changed. Fat treasure door Ahaobai, after and fat treasure sister bold English (] act the role ofing) fell in love, he brave and thunder without two; ALE when police, married and had and Arab affair girl Fifi (Athena ornaments). In order to let the vanity of his wife to live a happy life, he did not hesitate to choose corruption. Two black and white road, a good brother, work together to fight out of a world of their own......