Once upon a time, Saitama (voiced by Shin Furukawa) was just an ordinary office worker. One day, he met a boy who was attacked by a mutant crab by chance. A sense of justice rose in his heart and he rescued him. After that, Saitama's dream of being a hero in his childhood was revived in his heart. In order to become stronger, Saitama began hard training. In a blink of an eye, a few years passed, and he seemed to have gained extremely powerful strength as if he had been reborn, but at the cost of becoming bald. Because Saitama's power is so strong, no matter how terrible the enemy is, only one punch can determine the outcome. After that, Saitama met a modified person named Genos (voiced by Kaito Ishikawa), who worshipped Saitama as his teacher after appreciating his absolute strength. At the same time, Saitama and Genos signed up to join the Hero Association and became truly famous heroes, and thus began their journey to fight crime.