On a supercontinent Earth, powerful monsters and villains have been wreaking havoc in the cities. In response, 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. He has trained himself to the point of being able to defeat any enemy with a single punch, but his unmatched strength has left him with an overwhelming sense of boredom. Saitama eventually becomes a reluctant mentor to Genos, a cyborg seeking revenge against another cyborg who killed his family and destroyed his hometown. Saitama and Genos join the Hero Association, but due to scoring low on the written entry exam, Saitama is placed at a low-entry rank, and his feats remain unnoticed and unappreciated by the public.
The Hero Association calls all the top heroes to a meeting, informing them that the seer Shibabawa had a vision that the world is in danger before dying. Immediately after, an alien named Boros invades the planet. The heroes fight the invaders outside Boros’s ship, while Saitama boards the ship and fights Boros, who notoriously survives more than one punch from Saitama before being defeated. Saitama meets other heroes, such as the martial artist Bang, the esper Blizzard, and King, a hero considered the strongest man on Earth. Monsters also begin to appear at a rising rate. Another growing threat is the rogue martial artist Garo, Bang’s former apprentice and self-dubbed “Hero Hunter” Garo, who fights and defeats heroes to grow stronger and become a “monster”.
The monster influx is revealed to be the doing of the Monster Association, an organization composed of monsters intent on destroying the Hero Association. They attack various cities, kidnap a Hero Association executive’s child, and recruit fighters by offering them “monster cells”, which cause humans to mutate into monsters with extraordinary abilities. Throughout all this, Garo bonds with a child who idolizes heroes, seeks out heroes to battle, and greatly improves his fighting abilities. The Monster Association attempts to recruit him, and kidnaps the child when Garo refuses. Garo enters the Monster Association headquarters in City Z to rescue the child, but is captured. The Hero Association also breaches the headquarters to rescue the executives’ child and a chaotic battle ensues, in which City Z is destroyed. The majority of the Monster Association’s leaders are slain, but many of the heroes, overconfident in their abilities, are defeated. Garo, having achieved tremendous power, defeats the remaining heroes, before Saitama in turn arrives and defeats him. Despite the objections of the other heroes, Saitama spares Garo’s life and lets him flee.
In the aftermath of the battle, the Hero Association’s poor performance leads to public opinion of them plummeting. Several heroes and officials decide to retire or defect to the Neo Heroes, a burgeoning rival group that appears to be more effective in handling the growing monster threats with its larger membership and the leadership of the hero Blue, who claims to be the son of the long-missing Blast.