In a chaotic world plagued by monstrous threats, superheroes are classified and regulated by the Hero Association, battling everything from mutant beasts to alien invaders. Among them, a mysterious and unassuming man named Saitama lives in obscurity in City Z. Bald, emotionless, and dressed in a yellow jumpsuit, he is unlike any other hero, as he can defeat any enemy with a single blow.

Once an ordinary office worker, Saitama trained obsessively for three years, acquiring immeasurable power. But his overwhelming strength has left him bored and distant, yearning for a true challenge that never comes. When the city is attacked by an unusually coordinated series of monsters, Saitama intervenes, saving countless lives but receiving little recognition. He soon meets Genos, a ferocious young cyborg with a thirst for revenge.

Impressed by Saitama’s power, Genos becomes his disciple. Together, they join the Hero Association and face off against eccentric and powerful heroes such as the psychic Tatsumaki, the martial arts master Bang, and the vain superstar Amai Mask.

Faced with the emergence of a new threat—a powerful villain known as Lord Boros, who claims to be the strongest being in the universe—the world’s mightiest heroes are defeated one by one. When Boros arrives to destroy the Earth, Saitama must face an opponent who may finally surpass him with a single blow.