Saitama is a guy who's a hero for fun. After saving a child from certain death, he decided to become a hero and trained hard for three years.
A group of battlesuit-clad terrorists called "The Paradisers" appears in City F, and targets the fat-cat Zeniru, who in turn dispatches his bodyguard to defeat them.
Though he's been doing the hero gig for three years, Saitama realizes that the public knows nothing about him. Determined to get a little recognition, he decides to become a pro hero.
When Saitama learns that C-class heroes who don't fight bad guys are dropped from the hero registry after a week of inaction, he heads out to find any criminals or monsters to stop.
An enormous meteor hurtles toward City Z, forcing the Hero Association to ask its S Class heroes to respond.
Saitama and Genos head to the shores of City J to stop a pack of monsters calling themselves the "Clan of the Seafolk".
As the battle with the Deep Sea King reaches its climax, more heroes fall to the aquatic despot's brutal attacks.
Saitama tags along to an emergency meeting of Class S heroes and discovers that a seer has foretold a serious danger threatening the Earth.
The inconceivably powerful Lord Boros has traveled untold distances in search of someone who can cure his boredom: Saitama!