Saitama has returned to his ordinary life in Z-City after defeating Boros and his army. However, monsters are still appearing frequently, putting a toll on the Hero Association's resources. To counter this, they decide to recruit hoodlums, but during their first meeting, a mysterious man named Garou appears claiming to be a monster and starts attacking the crowd mercilessly. Garou continues his assault, defeating every hero he encounters. He is revealed to be Silverfang's former disciple and has unknown motives for his actions. Saitama, curious about this intriguing new foe and motivated by money, joins the martial arts competition where Garou is wreaking havoc. As the tournament progresses and Garou continues his rampage, a new threat emerges, potentially bringing about the catastrophic event predicted by Madame Shibabawa.