"Osu!! Karate Bu" centers on Kenji, a high school student who joins his school's karate club after transferring to a new school. Initially, Kenji is drawn to karate as a means to defend himself and gain confidence, but he quickly becomes immersed in the discipline and philosophy of the martial art. The series explores his growth as a karate practitioner, highlighting both his physical training and the mental challenges he faces. Kenji's journey is marked by his interactions with fellow club members, each of whom brings their own motivations and struggles to the dojo. The club is led by a strict yet supportive sensei who emphasizes the importance of respect, perseverance, and self-improvement.

The narrative is structured around Kenji's progression through various karate tournaments and sparring matches, which serve as milestones for his development. These competitions are not merely physical contests but also opportunities for the characters to confront their fears, insecurities, and rivalries. The series delves into the dynamics within the karate club, showcasing the camaraderie and occasional conflicts among its members. Key characters include Kenji's rival, a skilled but arrogant upperclassman who challenges Kenji to push his limits, and a female club member who balances her passion for karate with societal expectations.

As Kenji advances in his training, he begins to understand the deeper principles of karate, such as humility, respect for opponents, and the importance of continuous self-improvement. The series also touches on themes of perseverance, teamwork, and the balance between competition and personal growth. Through its focus on the karate club's activities, "Osu!! Karate Bu" provides a detailed portrayal of the martial art's techniques, traditions, and the dedication required to master it. The story concludes with Kenji reflecting on his journey, having gained not only physical strength but also a deeper sense of purpose and respect for the art of karate.