Lun (Jay Chou) lives with his father, Chiu (Anthony Wong), the music teacher of Tamkang Secondary School. Both of them are very good at music and piano. This is the first day Lun admitted to Tamkang Secondary School. When he walks through the campus with his classmates, a mysterious piano solo draws his attention. He follows the song and finds that an ethereal girl is playing piano in the old music room.

The piano girl is Yu (Guey Lun-Mei), one of Lun’s classmates. She is always late for school and hence sits next to Lun at the back row. The two become intimate friends and spend a lot of time together. « What’s the melody you played the first day we met? » asked Lun, « That’s my secret. » Yu whispered in his ear.

As the graduation day approaches, Yu decides to tell Lun her secret. She asks Lun to meet her at the music room that night. Lun replies her with a memo, « I will also tell you my secret tonight… »

That night, Lun plays Yu’s favorite melody while waiting for her, a hand covers Lun’s eyes. They kiss. But when Lun opens his eyes, he discovers that he was kissing Yi, who had fallen for him long time ago, his memo was mistakenly passed; Outside the music room, Yu saw everything and run away.

Yu never show up again. Lun asks his classmates, visits Yu’s home, but there is no trace of Yu. Even the student name list does not have Yu’s name. It seems that Yu has disappeared from this world…

One day, Lun gets a photo of Yu, but what surprises him is that Yu is standing next to his father on the photo! The secrets behind Yu and the mysterious melody are going to be unlashed…