物語開始から3年前、就職活動に行き詰まっていた青年・サイタマはある日、街で暴れていた怪人から1人の少年を救う。その際に「ヒーローになりたい」という幼き日に見た夢を思い出し、就活をやめてヒーローになることを決意。頭髪全てを失うほど懸命なトレーニングを3年間行った結果、どんな敵でも一撃で倒せる最強の力を手に入れる。
しかし、いつも一撃で決着が付いてしまうことから次第に戦いに対する緊張感などを喪失し、ヒーローになった現在でも無気力な日々を送っていた。
Saitama is a hero who only became a hero for fun.
After three years of "special training," he's become so strong that he's practically invincible. In fact, he's too strong—even his mightiest opponents are taken out with a single punch. Alongside Genos, his faithful disciple, Saitama performs his official hero duties as a member of the Hero Association.
One day, monsters claiming to be from the Monster Association suddenly appeared, taking a child of the Hero Association executive hostage. The S-class heroes gather and plan a raid on the Monster Association hideout to rescue the hostage. Meanwhile, Garou, a "human monster" who was taken by the Monster Association during a battle with the heroes, awakens in the Monster Association hideout.
