Mongkok, one of the most densely populated areas in the world, is the hotbed of illicit business. Lai Fu (Daniel Wu), a farm boy from Mainland China, is hired as a killer and is forced to enter into the coincidence of gangs of Mongkok. Lai Fu by chance meets a call girl from his home village, Dan (Cecilia Cheung), and saves her from the gangsters. The arrival of Lai Fu tenses the police and the triad. Under flashy neon lights, Lai Fu and Dan are rummaged around in Mongkok by police and gangsters. Who will find them first, the lawful force or the lawless one?