主人公埼玉原本是一名整日奔波于求职的普通人。3年前的一天偶然遇到了要对淘气少年下杀手的异变螃蟹人后，回忆起年少年时“想要成为英雄”的梦想，最终拼尽全力救下了淘气少年。之后通过拼命锻炼，埼玉终于脱胎换骨获得了最强的力量，但同时失去了头发成了光头。在独自做了一段时间英雄后，正式加入英雄协会，与众多英雄一起开始了对抗各种怪人以及恶势力的生活? ------ After rigorously training for three years, the ordinary Saitama has gained immense strength which allows him to take out anyone and anything with just one punch. He decides to put his new skill to good use by becoming a hero. However, he quickly becomes bored with easily defeating monsters, and wants someone to give him a challenge to bring back the spark of being a hero. Upon bearing witness to Saitama's amazing power, Genos, a cyborg, is determined to become Saitama's apprentice. During this time, Saitama realizes he is neither getting the recognition that he deserves nor known by the people due to him not being a part of the Hero Association. Wanting to boost his reputation, Saitama decides to have Genos register with him, in exchange for taking him in as a pupil. Together, the two begin working their way up toward becoming true heroes, hoping to find strong enemies and earn respect in the process.