What happens when a superhero can beat the snot out of every villain with just one punch? Can he find an opponent to give his life meaning, or is he doomed to a life of superpowered boredom? Nothing about Saitama passes the eyeball test when it comes to superheroes, from his lifeless expression to his bald head to his unimpressive physique. However, this average-looking guy has a not-so-average problem,he just can't seem to find an opponent strong enough to take on. The human monster and hero hunter Garo cranks up the intensity of his destruction.

As the damage increases, throwing the hero world into turmoil, Saitama decides it's the perfect time to develop an interest in martial arts and sneaks into a combat tournament. Meanwhile, the Class-S hero Metal Bat takes an assignment guarding a Hero Association executive and his son, and it isn't long before trouble appears!