When the Ronin Gang robs an armored car, three cops — Chan, Fong and Wai — go on the warpath. Each has a personal interest in bringing down Ronin leader Tien: he killed Chan's fiancée, humiliated Fong and kidnapped Wai's brother.

An anonymous love letter addressed to P5 student Lam Siu-wai throws her and her classmate So Hoi-shing into turmoil. The duo decides to find out who wrote the letter. Thus begins a journey whereby the two kids get their first taste of many things unknown to them.