"Hinomaru-Zumō" centers on Ushio Hinomaru, a determined high school student with a passion for sumo wrestling. Despite his small stature, Ushio dreams of becoming a Yokozuna, the highest rank in professional sumo. The story begins with Ushio enrolling at Oodachi High School, which has a struggling sumo club on the verge of disbandment due to lack of members. Ushio’s arrival revitalizes the club, as his unwavering dedication and skill inspire others to join, including his classmates and rivals.

The narrative follows Ushio and his teammates as they compete in high school sumo tournaments, facing off against formidable opponents from other schools. Each match is a test of physical strength, strategy, and mental fortitude, with Ushio often relying on his unique fighting style and indomitable spirit to overcome larger and more experienced wrestlers. Key rivals include Chihiro Kunisaki, a prodigious sumo wrestler with a calm demeanor, and Shinya Ozeki, a powerful and aggressive competitor. These rivalries push Ushio to grow both as a wrestler and as a person.

As the story progresses, the Oodachi High sumo club evolves from a ragtag group of novices into a cohesive team capable of challenging elite sumo schools. The series delves into the personal struggles and motivations of its characters, exploring themes of perseverance, teamwork, and the pursuit of greatness. Ushio’s journey is marked by setbacks and triumphs, as he learns to balance his fiery determination with the discipline required to excel in sumo.

The manga also provides insight into the traditions and techniques of sumo wrestling, offering a detailed portrayal of the sport’s physical and cultural aspects. Through intense training sessions, strategic matchups, and emotional character development, "Hinomaru-Zumō" captures the essence of sumo as both a competitive sport and a way of life. The series culminates in Ushio’s pursuit of the national high school sumo championship, a goal that tests the limits of his abilities and resolve.