Ye Xiang Lun is a music student majoring in piano who just transferred to Tamkang, a school famous for its musically talented students, especially those who play piano. On his first day of school, he hears a mysterious melody being played, and following it meets Lu Xiao Yu, 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 form a friendship that quickly evolves into a romantic relationship. However unbeknownst to Xiang Lun, there is more to Xiao Yu than initially meets the eye. 秘密奏鸣曲，清纯战胜刻薄。<<不能说的秘密>>是部完美的电影，关于电影疑点的全部解释。只说路小雨。只是关于桂纶镁。不能说的秘密等级考试，看你看懂了多少？。琴若流水，情如茉莉——关于《不能说的秘密》。不能说的秘密，一次华美的泄露。周杰伦式天马行空的略烧脑青春影片。不能说的秘密。只说桂纶镁。 叶湘伦（周杰伦 饰）是淡江艺术高中的一名插班生，父亲（黄秋生 饰）是该校的一名教师。这天同学晴依（曾恺玹 饰）带他参观学校，接着他独自来到了学校的旧琴房，本身弹得一首好曲的他被某处传来的一首钢琴曲吸引住了。他来到其中一间旧琴房，遇到了路小雨（桂纶镁 饰），纯纯的爱情故事由此发生了。一直以来，叶湘伦都觉得小雨充满了神秘，却无法进一步了解她。叶湘伦约小雨到琴房见面，叶湘伦以为晴依是小雨，跟晴依接吻时被小雨看到，自此之后叶湘伦就没有再看到小雨。叶湘伦到处寻找小雨，发现了那个小雨所说“不能说的秘密”的全部真相。
