After saving a child, Saitama trains for years to be a hero, becoming so strong he can defeat any opponent with a single punch.

The mysterious organisation known as The House of Evolution creates a monster that moves toward City Z; Genos, a young cyborg, attempts to stop it, but gets in over his head before Saitama happens along.

Saitama and Genos find out about the House of Evolution from Armored Gorilla and learn that Genus wants Saitama for his experiments.

A group of battlesuit-clad terrorists called The Paradisers appears in City F, and targets the fat-cat Zeniru, who in turn dispatches his bodyguard to defeat them.

Saitama decides to become a professional hero and takes up the test together with Genos. He barely passes while Genos achieves full marks.

Saitama has become a professional hero, but has yet to do anything heroic; Genos informs him that C-class heroes who don't fight bad guys are dropped from the registry after a week of inaction.