In a world overrun by monsters, Saitama seeks a thrill. Not from the monsters themselves, but from a good fight. The problem? Saitama has trained himself to such an incredible degree that he can obliterate any opponent with a single punch. This unmatched strength has turned heroism into a chore, leaving Saitama bored and yearning for a challenge.
Enter Genos, a cyborg with a vendetta against a powerful machine. Genos is awestruck by Saitama's power and begs to be his disciple. Saitama, unimpressed but finding a kindred spirit in Genos' determination, accepts. Together, they tackle monstrous threats and navigate the quirky world of the Hero Association, an organization that ranks and employs heroes.
Despite his immense power, Saitama struggles for recognition. The Hero Association, oblivious to his one-punch prowess, assigns him a low rank. Through hilarious battles and encounters with eccentric heroes, Saitama searches for an opponent who can match his strength and reignite his passion for fighting.