What is the plot of One-Punch Man?

On a supercontinent version of Earth that has four Moons, powerful monsters and supervillains wreak havoc. The millionaire Agoni creates the Hero Association, which employs superheroes to fight evil. Saitama, an unassociated hero, hails from City Z 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.

One-Punch Man is a manga and an anime about a protagonist, Saitama, who can defeat all of his enemies with one punch. It takes place on a version of Earth with one supercontinent and 4 moons, which has regular monster and supervillain attacks. To combat this, a Hero association was formed, but the protagonist is not part of this association and simply performs heroic deeds because he was bored. One of the highest ranked heros of the Hero association, Genos, sees Saitama's strength and becomes his disciple after Saitama beat a monster that beat Genos.