Euphonium player Kumiko Oumae inadvertently creates a rift with trumpeter and fellow band member Reina Kousaka upon losing their final concert band competition in junior high. On the first day of high school, she befriends classmates Hazuki Katou and Sapphire “Midori” Kawashima. Midori, who used to play the double bass, suggests that the three go check out the Kitauji concert band club. As they are being greeted by club president Haruka Ogasawara and vice president Asuka Tanaka, Kumiko is shocked to meet Reina again as the latter goes to join the band. While struggling to come to terms with her earlier surprise meeting with Reina, Kumiko runs into childhood friend Shuichi Tsukamoto on the way home, who also tells her of his intentions to join. Though initially unsure, with encouragement from Midori and Hazuki (who has just bought a mouthpiece), Kumiko decides to finally join the band the next day.