What is the plot of One-Punch Man?

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.