A single man arose to face the evil threatening humankind! His name was Saitama. He became a hero for fun!
With one punch he has resolved every crisis so far, but no one believes he could be so extraordinary strong.
Together with his pupil Genos (Class S), Saitama has been an active hero and risen from Class C to Class B.
One day, a man named Garou shows up. He admires monsters so he begins hero hunting. During a monster outbreak that is wreaking havoc, Garou is fighting metal Bat, when elsewhere Fubuki faces the monster known as Do-S.
Meanwhile, Saitama has cleared the first round of the martial arts tournament!