Saitama is a superhero who has trained so hard that his hair has fallen out, and who can overcome any enemy with one punch. However, because he is so strong, he has become bored and frustrated that he wins all of his battles too easily.
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. Though he’s now so strong he can defeat any opponent with a single punch, lately he feels as if, in exchange for overwhelming power, he’s lost something even more important.
A monster is moving toward City Z, one that has been created by the mysterious organization known as The House of Evolution. Genos, a young cyborg, attempts to stop it, but gets in over his head before Saitama happens along.
The narration describes the story of Doctor Genus, brilliant young scientist with ideas of artificially evolving humanity becoming disillusioned with the world, eventually discovering the secret to immortality and creating multiple clones of himself. Saitama cuts off the narration in his impatience and Armored Gorilla then explains that Doctor Genus wants Saitama for his experiments. Genos and Saitama decide to attack immediately, since the store discount day is the next day. Before they leave, Genos asks Armored Gorilla about any cyborgs active four years ago that have destroyed any towns, which Armored Gorilla denies. In preparation for Saitama's arrival, the House of Evolution sets traps in their facility and reluctantly decide to release Asura Rhino. Once they arrive, Genos destroys the above-ground facility with a single blast before heading underground. The released Asura Rhino fights with Genos, taking no damage in the process. Asura Rhino then faces off against Saitama, but backs off when he detects how powerful Saitama really is. Saitama then reveals to the room his somewhat mundane daily training regime of 100 push-ups, 100 sit-ups, 100 squats and 10 km of running, much to Genos's disbelief. Asura Rhino unleashes his Asura Mode and appears to be beating Saitama back, but it transpires that Saitama was just shocked by the realization that the store discount day was that day instead of the next and defeats Asura Rhino easily, leaving soon after with Genos to catch the rest of the discount day. Having witnessed this, Doctor Genus abandons his plans of artificial evolution. After the credits, officials from the Heroes Association inspect the House of Evolution blast site and discuss Genos's destructive power.
A group of battlesuit-clad terrorists called "The Paradisers" appears in City F, led by an enormous man known as Hammerhead. Calling for the redistribution of wealth, the group targets the fat-cat Zeniru, who in turn dispatches his bodyguard to defeat them. Unfortunately, Saitama is on the same errand.
Saitama and Genos find out about the hero registration and both take the tests to certify. Genos gets "S(Special)" and Saitama gets a "C" grade. Meanwhile, Genos became the disciple of Saitama and learned about the power of saitama's punch on a training session.
It's been five days since Saitama became a professional hero and he has yet to do anything heroic. When Genos informs him that C-class heroes who don't fight bad guys are dropped from the hero registry after a week of inaction, Saitama heads out on patrol, yet can't 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. When all their efforts prove in vain, Genos decides it's up to him to protect the home of his master and stop the meteor, no matter what the cost.
A pack of monsters calling themselves the "Clan of the Seafolk" arrive on the shores of City J intent on invading the land. When the hero sent to fight them is defeated easily by the Deep Sea King himself, Saitama and Genos head for the city to help.
The Deep Sea King has defeated every hero who opposed him with ease. Genos is preparing to attack while Licenseless-Rider and Saitama rush to the evacuation shelter, where the battle continues.
Saitama tags along to an emergency meeting of Class S heroes and discovers that a seer has foretold a serious danger threatening the Earth. Though the crisis could occur anytime in the next six months, the heroes are surprised to hear the roar of explosions that very hour!
As a colossal spaceship floats in the skies above City A, a member of its monstrous crew battles on the ground against a group of S Class heroes. Despite their powers, they can't seem to finish it off. Meanwhile, Saitama has boarded the enemy ship on his own.
The inconceivably powerful Lord Boros has traveled untold distances in search of someone who can cure his boredom: Saitama! With the fate of humanity in the balance, will the two champions at last find in each other the worthy opponent they've been searching for?