"Sound! Euphonium" centers on the Kitauji High School Concert Band, a group striving to overcome past mediocrity and achieve excellence in national competitions. The narrative begins with Kumiko Oumae, a first-year student and euphonium player, who joins the band after a lackluster middle school experience. Initially ambivalent about her musical passion, Kumiko’s perspective shifts as she becomes immersed in the band’s rigorous practices and interpersonal dynamics.

The band’s new advisor, Noboru Taki, a former alumnus with high expectations, challenges the group to aim for the national competition. His strict leadership and focus on discipline create tension but also inspire growth. The story explores the struggles of balancing personal ambition with collective goals, as students grapple with self-doubt, rivalry, and the pressure to perform. Key characters include Reina Kousaka, Kumiko’s talented but enigmatic classmate, whose dedication to the trumpet and pursuit of perfection influence Kumiko’s own journey. Hazuki Katou and Sapphire Kawashima, Kumiko’s close friends, provide emotional support and comic relief, while upperclassmen like Asuka Tanaka and Kaori Nakaseko navigate their own challenges as section leaders.

The series delves into the complexities of teamwork, highlighting both the camaraderie and conflicts that arise within the band. Themes of perseverance, identity, and the pursuit of artistic excellence are woven throughout the narrative. As the band progresses through auditions, rehearsals, and competitions, the characters confront their insecurities and grow both as musicians and individuals. The story culminates in their performance at the Kyoto Prefectural Concert Band Competition, where their hard work and determination are put to the test. "Sound! Euphonium" captures the emotional highs and lows of high school life, offering a nuanced portrayal of the challenges and rewards of striving for greatness in a competitive environment.