Saitama is a hero who only became a hero for fun. After three years of “special” training, though, he's become so strong that he's practically invincible. In fact, he's too strong — even his mightiest opponents are taken out with a single punch, and it turns out that being devastatingly powerful is actually kind of a bore. With his passion for being a hero lost along with his hair, yet still faced with new enemies every day, how much longer can he keep it going?
An increase in villain activity has the Hero Organization worried that the "Earthdoom prophecy" will soon come to pass. In an effort to alleviate the overworked heroes, they turn to villains themselves for help. The decision looks ill-advised as at least one villain is more interested in helping the prophecy along than preventing it.
