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 the seer Shibabawa predicts a great danger to Earth and dies. Soon after, a group of interstellar bandits named Dark Matter Thieves, led by an alien named Boros, attack Earth. Class S heroes and Saitama defeat the alien invasion, but A-City is destroyed. Later, the Hero Association erects a fortress where the city once was, and a man named Garou, who admires monsters, shows up...