One Punch Man Season 2 VOL.1-12 End
On an unnamed Earth-like super-continent planet, powerful monsters and villains have been wreaking havoc in the cities. To combat them, the world’s government created a Hero Association that employs superheroes to stop them. The heroes are ranked from Class C to Class S. Saitama, an unassociated hero, hails from the metropolis of City-Z and performs heroic deeds for his own entertainment. He has trained himself to the point where he broke his “limiter” and can effortlessly defeat any opponent with a single punch, but he is now bored with his own omnipotence and frustrated from his lack of sense of challenge.
故事發生於一個類似於地球，但有著超能力與各種奇怪生物的星球。各種具有超自然力量的「怪人」不斷地出現並造成災難，為了打擊它們，由某富豪所創立的「英雄協會」開始招募全世界的能人異士成為職業英雄與怪人對抗。
故事主角埼玉便是一名英雄，他具有無敵的力量，但因為能力太過強大，卻沒了當初能打敗怪人那激昂的情緒，只有在他遇見有挑戰性的對手時才能興奮起來。在一連串事件中，埼玉遇見了各式各樣的英雄、反派和怪人，收了改造人傑諾斯做為徒弟，並加入英雄協會以獲得官方認可。與眾多英雄一起開始了對抗各種怪人以及災難的生活，但更多時是在無人知曉的情況下做英雄的工作。