Joe Yabuki has learned how to toughen up and stop trusting others after a troubling youth spent bouncing between orphanages and fending off bullies. After finally escaping the cycle of violent temporary homes, Joe adopts a delinquent lifestyle in the streets of San'ya, a slum in Tokyo. He makes his way through the world with his fists, picking fights with anyone he pleases.

Danpei Tange, a washed-up and alcoholic former boxing coach, notices Joe's animalistic combat ability and attempts to mentor the wild teenager. Despite developing an explosive relationship, the two form a close bond and rely on each other to look after the other homeless children in San'ya.

Unable to abandon his destructive habits, Joe is arrested for petty crimes. In jail, he meets and fights with lifelong friends and rivals, including the boxing prodigy Tooru Rikiishi. Continuously challenging Rikiishi to fights, Joe is knocked out time after time due to his savage brawling style failing in the face of Rikiishi's controlled boxing technique. But eventually, Joe stuns his rival with a knockout blow, earning Rikiishi's respect and the promise that they will one day meet again inside the professional ring.

Finding a reason to live within boxing, Joe takes life head-on with a new purpose. As he rises through Japan's amateur and professional boxing scenes, he contends with painful loss and increasing injury. Joe's passion for fighting is all-consuming, and he grapples with the idea that a life without boxing may not be one worth living.