Ye Xianglun (Jay Chou), a music student majoring in piano, transfers to Tamkang (Danjiang) Secondary School. It is a school famous for musically talented students, especially in piano. On the first day of school, as he wanders through the piano building, he hears a mysterious melody being played that leads him to Lu Xiaoyu (Gwei Lun-mei), another piano major. When he asks her about the song she was playing, she tells him that it is a secret that cannot be told. The two develop a relationship that is clouded in mystery. When Xianglun tells Xiaoyu about the demolition of the piano building on graduation day, she teaches him the mysterious song.
However, when a mis-passed note leads to Xianglun accidentally kissing another student Qing Yi (Alice Tzeng), Xiaoyu disappears for five months, only to return on graduation day and mysteriously disappear again. Xianglun asks around and discovers from Xiaoyu's mother and his father (Anthony Wong Chau-sang) that Xiaoyu was actually a student from the class of 1979, who graduated 20 years ago. His father tells him she claimed she traveled forward in time to the present with the help of a piano piece called "Secret" that she played on an old piano in the piano building. She had fallen in love with Xianglun, but the only barrier is that, in the present, the first person she sees on each trip is the only person that can see her.