In a quicksand surrounded town called Makka, rumors spread of a legendary robber named Gasback is after the town. To protect it, Mayor Kepler has hired bounty hunters. These hunters have been following Gasback from town to town in hopes of getting the bounty. Vash the Stampede is in town, along with Meryl and Milly, along with the female bounty hunter Ameria and Nicholas D. Wolfwood.

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?