Saitama looks like an average guy, but his problem is anything but average. After training hard enough for all his hair to fall out, he's become so overwhelmingly powerful that no villain can stop him. The thing is, Saitama just does the hero thing for fun. When every enemy goes down with a single punch, it turns out that overwhelming power can be kind of... boring. Can a hero be too strong?

Saitama, alongside his cyborg disciple Genos, is ready to begin his official duties as a professional hero. But when a new friendship and interest in martial arts prove distracting, the Hero Association is left to deal with a sinister new wave of monster attacks on their own. To make matters worse, their heroes are being hunted! Is Earth doomed, or will Saitama save the day with one 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 the 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.