Upon entering Kitauji High School, Kumiko Oumae yearns to leave her past behind. However, her plans for a fresh start are derailed when her new friends, Sapphire Kawashima and Hazuki Katou, rope her into joining the school's band club, where she is once again assigned to play the euphonium.
The band is currently ill-prepared for the upcoming local festival, Sunfest, let alone the prospect of competing at a national level. Their newly appointed advisor, Noboru Taki, presents them with a choice: they can either relax and enjoy themselves or diligently practice to strive for a spot in the national competition. Recalling her past mistakes in middle school, Kumiko is uncertain about pursuing nationals once again. Amidst the ensuing chaos, she discovers that her former bandmate, Reina Kousaka, with whom she shared a strained relationship, has also joined Kitauji's band club.
Under the demanding tutelage of Noboru, Kumiko and her bandmates must navigate their personal struggles and learn to work together to overcome challenges. As they face the pressure to excel, they must find a way to harmonize their talents and strive for collective success.