Yuri Kashiwaga is a talented musician who leaves Tokyo and returns to her hometown on the Goto islands. Her friend and a teacher in the local school goes on maternity leave and Yuri substitutes for her. She is also in charge of the school choir, which is preparing for a competition. Yuri gives her students a task to write a letter to themselves as if they were 15 years older. Thus, fifteen-year-olds reveal their current secrets and worries to the thirty-year-old versions of themselves.