Saitama is an anonymous office worker who resides in Z City, a fictional metropolis in Japan. After being fired, he decides to face the monsters and villains that plague the world. After a hard training that ends up causing him to go completely bald, he manages to become the most powerful hero on the planet, capable of finishing off any enemy with a single punch.

Saitama is a hero who only became a hero for fun. After three years of “special training,” 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. Now, the great seer Madame Shibabawa’s prediction about the Earth being doomed seems to be coming true as the frequency of monster incidents escalates. Alongside Genos, his faithful disciple, Saitama begins his official hero duties as a member of the Hero Association, while Garou, a man utterly fascinated by monsters, makes his appearance.

Saitama is a hero who only became a hero for fun. After three years of "special training," 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. Alongside Genos, his faithful disciple, Saitama performs his official hero duties as a member of the Hero Association. One day, monsters claiming to be from the Monster Association suddenly appeared, taking a child of Hero Association executive hostage. The S-class heroes gather and plan a raid on the Monster Association hideout to rescue the hostage. Meanwhile, Garou, a "human monster" who was taken by the Monster Association during a battle with the heroes, awakens in the Monster Association hideout.