"Sound! Euphonium: Ensemble Contest Arc" centers on Kitauji High School Concert Band as they prepare for the Kyoto Ensemble Contest, a smaller-scale competition that allows smaller groups within the band to showcase their skills. The arc focuses on the formation of smaller ensembles, highlighting the challenges and dynamics of working in more intimate groups compared to the full band. The narrative primarily follows Kumiko Oumae, a second-year euphonium player, as she navigates her role in a brass quintet alongside her peers.

The quintet, consisting of Kumiko, Reina Kousaka on trumpet, Hazuki Katou on tuba, Sapphire Kawashima on horn, and Midori Kasaki on trombone, faces internal struggles as they strive to harmonize both musically and personally. Reina’s perfectionism and Kumiko’s self-doubt create tension, while the group must also contend with the pressure of representing their school. The arc delves into the individual growth of each member, emphasizing their dedication to improving their craft and overcoming personal insecurities.

Parallel to the quintet’s story, other ensembles within the band also prepare for the contest, showcasing the diverse talents and personalities within Kitauji High. The narrative explores themes of teamwork, perseverance, and the pursuit of excellence, as each group grapples with their unique challenges. The arc culminates in the Ensemble Contest, where the students’ hard work and determination are put to the test, offering a satisfying resolution to their journey while setting the stage for future developments in the series.