The story follows the life of Saitama, a superhero who has become so powerful that he can defeat any enemy with a single punch. The series is set in a world where superheroes and villains exist, and follows Saitama's journey as he becomes a professional hero and joins the Hero Association, an organization that ranks heroes based on their abilities. Despite his incredible strength, Saitama is often overlooked and unappreciated because of his unimpressive appearance and lack of ambition.