Due to monsters and villains wreaking havoc in the cities, a millionaire Agoni creates an organization called Hero Association which employs superheroes to fight against villains and monsters to keep the cities safe. Saitama an unassociated hero from city Z who has trained himself to the point where he can effortlessly defeat any opponent, performs heroic deeds for his own amusement. He became too strong that he can defeat any opponents with a single punch. This makes him bored with his own omnipotence.

Saitama meets with a cyborg, Genos who is seeking revenge against another cyborg that killed his family and destroyed his hometown. Seeing Saitama’s strength, Genos decides to become Saitama’s disciple. Genos then moves in with Saitama. Even though he has saved countless people’s life he is not famous. Genos points out the reason that he is not a registered hero in the Hero Association. So, Saitama and Genos joins the Hero Association. They qualify the examination where Genos becomes S rank hero while Saitama becomes C rank hero. Saitama performs many feats that go unnoticed and unappreciated by public because he is a low ranked hero.

Shibabawa an old woman who has the ability to predict the future had a vision that the world is in danger. The Hero Association gathers all top heroes for helping the world from danger. Immediately after the meeting the Earth gets attacked by an alien named Boros. The heroes fights the invaders outside the Boros’s ship while Saitama infiltrates the ship. He finds Boros and defeats him. Saitama meets other heroes like, Bang, Fubuki and King. The hero King is considered to be the strongest man on earth meets. When a monster attacks King Saitama rescues him where King confess that he was accidently takes credit for Saitama’s deed.

Due to the increase in monsters, the association tries to recruit more heroes. At that time Bang’s former apprentice Garo begins beating heroes left and right ruthlessly, earning the title Hero Hunter. Saitama enters a martial arts tournament as Charanko, a disciple of hero Silver Fang. There he fights the previous champion Suiryu where Saitama effortlessly defeats him. But the tournament committee finds out that Saitama was a fraud and disqualifies him. During tournament Monster Association attacks the cities causing a havoc.

The monster association kidnaps one of the hero association executives and his child. During the fight monster association recruits fighters and offers them monster cells which mutates them to monsters giving them extraordinary powers. During the fight between Garo and his master Bang, where Garo is beaten down, the monster association interrupts them. The monster association kidnaps Garo as they want to appoint him.