一个在24小时以内的故事。发生为了寻找失踪女友，来福（吴彦祖饰）从内地移民至香港，在旺角找了一份差事，误入歧途成了一名杀手，卷入了黑社会的腥风血雨中。提携他的是奸诈的老六，他混迹于黑白两道，是警方的线人，来福因此踏上亡命之途。其间遇到妓女丹（张柏芝饰），相似的身世令二人相见如故，丹决心帮来福找回女友，同时要改过自身。 　　警方多次抓捕来福未果，眼看江湖恩怨渐渐平息，正打算就此作罢，来福和丹似乎可以喘一口气了，但是事情又出现了意外的转折，夜幕下的旺角始终处处暗涌。Hong Kong nihilism. December 22, a street quarrel leads to the death of a gang leader's son. Next day, he seeks revenge on his brother, a rival boss. He calls on Liu, a fixer, to import a hit man from the mainland. Lai Fu, a tough and youthful hick, arrives with a day pass. The cops, led by the morose Milo, hear about the killer; they open a full-scale Christmas Eve operation to find the warring brothers and Lai Fu. Lai Fu rescues a hooker, Dan Dan, from a sadist and asks her to help him find his way around Mongkok. By nightfall, Liu has double crossed Lai Fu, the brothers are hiding, the cops are everywhere, and Lai Fu and Dan Dan are on the run. Peace on earth, good will to all?