Chuuji Akagi is an orphaned seventh-grader who transfers into a school dominated by a youth gang. He lives with his grandfather, Takeshi Akagi, a master of jiu-jitsu. Despite his short stature, Chuuji possesses significant athletic ability and muscular strength, contrasting sharply with his poor academic performance. He is defined by a strong sense of justice and loyalty toward his friends. The death of his parents caused a specific psychological aversion, making him unwilling to engage in fights at sunset.

He challenges the school's gang structure by confronting its members through sports competitions instead of physical brawls. The original gang leader, Kojiro Kose, initially scouts rivals to oppose Akagi but is later inspired by his morals, transitioning to lead the soccer club while still creating indirect conflicts. Akagi defeats numerous adversaries, each excelling in different disciplines: Hiroshi Kosai, who nearly blinds him before reforming and becoming an ally; Yusuke Kurobe, a formidable former juvenile detention inmate; and Saburo Tenma, known for exceptional circus-trained grip strength. Other opponents include Kyogei Samegawa in karate, Suzumu Hayate in baseball, and Robert Douglas in soccer.

Akagi's influence inspires classmates. Academically gifted but physically limited Teruo Aoki joins the kickboxing club and later captains the table tennis team. Yoko Mizunoe, excelling both academically and athletically, joins the volleyball club and develops romantic feelings for him. An elementary school neighbor, Kantaro, intensely admires him, calling him "boss." Through consistent victories in these sports challenges, Akagi reforms the school environment, transforming it from gang rule to one focused on athletic discipline and camaraderie. His teacher, Mr. Araki, occasionally appears in subplots involving unrequited romantic interests unrelated to Akagi's central conflicts.