"ON-GAKU: Our Sound" centers on Kenji, a high school delinquent with a penchant for fighting and a lack of direction in life. Alongside his friends Asakura and Ota, Kenji spends his days aimlessly wandering and occasionally picking fights. Their lives take an unexpected turn when Kenji stumbles upon an old, discarded bass guitar. Intrigued, he decides to form a band, despite having no prior musical experience or knowledge. Asakura and Ota join him, with Asakura taking up the drums and Ota reluctantly agreeing to play the keyboard.

The trio’s initial attempts at making music are chaotic and unstructured, reflecting their inexperience and carefree attitudes. However, their raw enthusiasm and unconventional approach gradually draw attention from their peers. The band, named "Kobujutsu," begins to attract a small following, including a classmate named Morita, who becomes their manager and helps them navigate the challenges of performing live.

As the story progresses, the band’s journey becomes a metaphor for self-discovery and camaraderie. Kenji, initially motivated by boredom and a desire to stand out, begins to find purpose and passion in music. The narrative explores the dynamics between the band members, their struggles with self-doubt, and their growing determination to improve. Their performances, though unpolished, are marked by an infectious energy that resonates with their audience.

The climax of the story revolves around Kobujutsu’s participation in a local music festival. Despite their lack of technical skill, the band’s performance becomes a defining moment, showcasing their unique sound and the bond they’ve forged through their shared experiences. The film concludes on an open-ended note, leaving the future of the band uncertain but emphasizing the transformative impact of their musical journey on their lives.

The characters are defined by their simplicity and relatability. Kenji’s impulsive nature and gradual growth anchor the narrative, while Asakura’s laid-back demeanor and Ota’s reluctant participation provide both humor and depth. Morita’s role as the pragmatic yet supportive manager adds a layer of structure to the otherwise chaotic group dynamic. Together, they embody the spirit of youthful experimentation and the power of music to bring people together.