On "Earth" super-continent planet, powerful monsters and villains wreaks havoc in the cities. To combat them, the world's government created a Hero Association that employs superheroes to stop them. The heroes are ranked from Class C to Class S. Saitama, an unassociated hero, hails from the metropolis of City-Z and performs heroic deeds for his own entertainment. He has trained to the point where he broke his "limit" and can effortlessly defeat any opponent with a single punch.