Yuri Kashiwagi is a genius pianist and part-time teacher in Tokyo. When her former colleague Haruko goes on maternity leave, she asks Kashiwagi to take over for her as the choral advisor at a middle school on one of the Goto Islands in Nagasaki Prefecture. When Kashiwagi arrives, her first assignment for her new students is to write a letter to themselves 15 years in the future. The letters they write eventually reveal their secret worries and concerns about reaching adulthood.