After three years of intense training, Saitama, once an ordinary man, has attained overwhelming power, able to defeat any enemy with a single punch. Driven by a desire to use his new strength, he sets out on a path as a hero, but soon grows disillusioned by how easily he wins and the absence of any worthy opponent to challenge him.
The cyborg Genos, impressed by Saitama's incredible power, asks to become his apprentice. Saitama, however, realizes that because he is not registered with the Hero Association, he remains unknown and overlooked by the public. To gain more recognition, Saitama agrees to register alongside Genos, on the condition that Genos becomes his disciple. Together, they aim to climb the ranks of heroes, seek out powerful enemies, and earn the admiration and respect of others.