After defeating Boros, Saitama returns to his boring life in City Z. Meanwhile, the number of monsters is rapidly increasing, putting the Hero Association in crisis. Amidst the chaos, Garou emerges, a man who proclaims himself a monster and begins hunting heroes without mercy. A former disciple of the legendary Silverfang, he challenges everything and everyone with frightening strength. Intrigued by this new opponent, Saitama decides to participate in a martial arts tournament, unaware that an even greater threat is about to put all of humanity at risk.

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.