Saitama is a man who started to be a hero because of his hobby. After 3 years of special training, he gained invincible power and became a hero who could defeat any enemy with just one blow. Together with Genos, who became his apprentice by chance, he began to officially carry out heroic activities in the Hero Association. As the incidence of monsters increased abnormally, the Hero Association thought of the possibility of the disaster "The Earth is Over" left by the great prophet Wrinkled Granny, and prepared to start researching countermeasures. At this time, Garou, who admired monsters, appeared.