Life gets pretty boring when you can defeat every villain with just one punch. Nothing about Saitama passes the eye test when it comes to superheroes – from his expressionless face to his bald head and his unremarkable physique. Yet this average-looking guy has a not-so-average problem: he just can't find an opponent strong enough to face him! The monster Goketsu appears at the martial arts tournament and tries to transform the participants into monsters by giving them monster cells. Tournament champion Suiryu fights one after another against the newly transformed fighters until a certain hero rushes to his side to help!.