Plot of the story
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.
When Xiaoyu saw Qing Yi kiss Xianglun, she returned to her normal time. During her absence at home, her story spread and everyone believed that she was mentally ill. Remembering that the piano room would be destroyed on graduation day of 1999, she tried time traveling to see Xianglun one last time, but she spotted Xianglun wearing Qing Yi's bracelet. Believing that Xianglun now belonged to Qing Yi, Xiaoyu returns to her own time with a heavy heart. Due to an asthma attack, while writing on her desk trying to leave a message to the boy she loves, Xiaoyu dies. Xianglun sees the writing on the desk and desperately tries to write back usingwhite out but when he gets no response he rushes to Xiaoyu's house only to find her room empty. Xiaoyu's mother finally recognises Xianglun as the boy her daughter had drawn a picture of twenty years before and Xianglun learns the truth about her time travels.
After finally learning of Xiaoyu's story, Xianglun realizes that the piece that she taught him was "Secret", which had the power to take the pianist forward or backward in time depending on the tempo at which it is played. He rushes off to the piano room, which is about to be torn down. Xianglun's father retrieves the manuscript of the musical piece that Xiaoyu had given to him for safekeeping, then realises that Xiaoyu had written a secret message for Xianglun. Meanwhile, Xianglun enters the piano building and as the demolition begins, Xianglun starts to play "Secret" from memory, recalling something Xiaoyu told him while teaching him the piece: "I always play it that fast when I go back". Just before the piano room is completely destroyed, Xianglun travels back to 1979. He sees Xiaoyu, and she sees him and smiles. The last scene is when the 1979 Tamkang (Danjiang) Secondary School graduation picture is taken, in which Xianglun and Xiaoyu are present. As the old piano and the piano building have been destroyed, Xianglun stays forever in the past with Xiaoyu.
