Yuri Kashiwaga, formerly celebrated as a brilliant pianist, decides to return to her roots in the Goto Islands after living in Tokyo. She takes on a role as a substitute teacher at a local middle school, where she also becomes the advisor for the school choir. The choir is preparing to compete in an upcoming contest. To inspire the students, Yuri assigns them a task: to pen a letter titled “To Myself 15 Years in the Future.” Through this exercise, the 15-year-old students reveal their personal secrets and concerns.