Once, two men fought for the top of Japanese boxing. One died in the ring after the match; the other, determined to raise a world champion, moved to America and later died of illness. Years later, the survivor's son, Dan Hayasaka, returns to Japan from an American monastery. He comes to live with the children of the man his father fought—Asahi and Misuzu Inoue—who run a struggling boxing gym in their deceased father's name. Dan speaks broken Japanese, is deeply religious, and refuses to harm even an insect, but his left-handed punches possess supernatural power that he believes was given by God. He vows to use that power to become the world boxing champion and fulfill his father's dream.

Dan's journey begins with a debut match against Kitamura Tomoki, a fearsome brawler known as the "ring killer" who has a traumatic past. Dan's refusal to fight unfairly opens Tomoki's heart, turning a bitter rival into a lifelong friend and eventual rival for the world title. After rejecting a lucrative offer from the powerful Raitoin Gym, a powerful old woman who controls the bantamweight division, Dan faces a series of opponents sent to crush the small gym's pride. Each opponent has a personal tragedy, and Dan's compassionate style—honoring his father's code of fair play—often redeems them even in defeat.

Dan climbs the Japanese ranks, defeating former champion Munakata Zenzou and capturing the national bantamweight title from the arrogant Himuro Taira, whose "phantom punch" is too fast for the eye. From there he travels to America, where he meets Duran Michael, a childhood acquaintance now a world-ranked boxer, and later challenges the undefeated world champion John Phoenix. Dan's world title victory opens the path to a unification bout with now-world-champion Kitamura Tomoki. In the final arc, Dan moves up in weight to the same lightweight class his father fought in, and there he discovers a long-lost half-brother—the son of his father's estranged wife—with whom he reconciles before a final match. Dan becomes the unified world champion, and the story ends with him praying, "Thank you, God." The series emphasizes human drama over blow-by-blow ring action, with each match preceded by deep exploration of the fighters' personal histories.