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.

The third season of One Punch Man. 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.