高中生葉湘倫（周杰倫飾）出身單親家庭，並且在父親 （黃秋生飾）任教的學校就讀。而在父親的耳濡目染下， 他熱愛音藥並且琴藝過人。某日，班上來了一位同樣 喜愛彈鋼琴的新同學路小雨（桂綸美飾），投緣的兩人 形影不離，情感也日漸加溫，然而小雨總是相當神秘， 還常彈奏一首未曾問世，但優美動聽的曲子。而每當 小倫想多了解小雨一些，她常欲言又止，只都推說是 秘密。但有一天，在一場誤會發生後，小雨再也沒來上 過課，思念小雨又一頭霧水的小倫決心要找出這個不 能說的秘密。。。

Jay (Jay Chou) is a high school student from a single parent family. His father (Anthony Wong) holds a post as a teacher in his school. The influence of his father passion toward music made him loves music and good at playing piano. One day, a new student, Rain (Kwai Lun Mei) came to study in his class. Rain loves playing piano as much as he does. Both of them have a great rapport and always stay together. However, Rain is a mysterious girl. She always plays a melodious song that never been heard before. Every time when Jay wishes to know more about her, she would falter and say it's a secret. After a misunderstanding happened, between them, Rain never comes to school again. With yearning and suspicions, Jay has made up his mind to find out the untold secret...