Saitama is a guy who's a hero for fun. After saving a child from certain death, he decided to become a hero and trained hard for three years. Though he's now so strong he can defeat any opponent with a single punch, lately he feels as if, in exchange for overwhelming power, he's lost something even more important.

Saitama and Genos track down an evil scientist who wants to know the secret to Saitama's power.