Set at a suburban high school, the story follows Kumiko Oumae, a reserved euphonium player who reluctantly rejoins her school's concert band and becomes absorbed in its drive to reach the national competition. As the ensemble grapples with leadership changes, clashing personalities, and the pressure of past achievements, Kumiko develops a complex friendship with the brilliant, intense trumpeter Reina Kousaka while other members confront their own doubts, ambitions, and rivalries. Through rehearsals, performances, and quiet everyday moments, the novel examines coming of age, the discipline and joy of ensemble music, and how sound shapes identity and relationships.