In the aftermath of defeating Boros and his powerful army, Saitama has returned to his mundane life in Z-City. Despite this, the number of monsters appearing continues to increase, stretching the Hero Association’s resources thin. To combat this rising threat, the association makes a controversial decision to recruit criminals to help fight. However, during their first meeting with these potential recruits, a mysterious man named Garou suddenly appears, declaring himself a monster and brutally attacking everyone present.

Garou, a former top disciple of the martial arts master Silverfang, continues his violent rampage against the Hero Association, defeating hero after hero. His motives remain unclear, but his strength and relentless drive make him a formidable adversary. Intrigued by the enigmatic Garou and always on the lookout for money, Saitama decides to enter a martial arts tournament, eager to see what all the fuss is about.

As the tournament progresses and Garou’s attacks escalate, a far greater threat looms over humanity. This new menace may be the earth-shattering catastrophe foretold by the great seer Madame Shibabawa, leaving the fate of the world hanging in the balance.