Saitama, an unassociated hero, hails from Z-City and performs heroic deeds as a hobby. For three years, he has trained enough to defeat any enemy with a single punch, his unmatched strength leaving him bored. He becomes a reluctant mentor to Genos, a cyborg seeking revenge against another cyborg who killed his family and destroyed his hometown, after Saitama defeats a monster that defeated Genos. With Genos, Saitama learns about a heroes organization who are also fighting the monsters.