A story set sometime between Mosquito Girl's defeat and Genos showing up to Saitama's apartment. Genos wants to learn the secret to Saitama's power and decides to follow him and to observe everything he does. At the same time, Saitama is convinced that someone is stalking him.

Saitama gives Genos a key to their apartment and they both mull over items to buy for dinner; however, upon remembering a limited time sale at two separate supermarkets, the pair split up. Genos then senses a monster at a nearby shopping strip and defeats it. However, this makes him miss one of the sales. Once he reaches the other (closed down) supermarket, Genos bumps into the Dai Fuu Family's No. 2 man and the rival C-Class Pacino Family, headed by Don Pacino. Pacino mistakenly believes Genos is associated with the Dai Fuu Family; he steals the apartment key and kidnaps both men, promising to let them and the keys go once Genos states his case. Once Genos finishes telling his worthless story to the Pacino family, Don Pacino becomes angry with Genos and the family opens fire. Genos easily defeats them and takes his key and the Dai Fuu Family's No. 2 man with him. After explaining his tardiness to Saitama, the pair enjoy some sukiyaki.

After the events of episode 4, Sonic strive to strict training muffled to the mountain, but had spent the days of troubled with nightmares to be a nasty blow to the groin from Saitama who suffered a defeat. Man of Matagi had seen such a situation. He is also "formidable enemy" in defeated in the past of the owner ....

After the events of episode 7, Bang begins to spy on Saitama and Genos to gauge their abilities. Hopeful that his Water Stream Rock Smashing Fist martial arts is stronger, and that proving so will coerce Saitama and Genos to study under him, Bang invites Saitama and Genos to his dojo under the pretense of giving them free meat.

During a coincidental train ride, Genos and Fubuki are put to the test when an anti-Hero Association group plants a bomb on the train.

Child Emperor investigates the murder of Zombieman.

Saitama meets a group of villains in a public restroom who mistake him for their ally, they introduce themselves to Saitama, but when he reveals he's a hero, attempt to defeat him.

Atomic Samurai and Bang go out fishing to a lake. Giant Salamander attacks the lake and Chain'n'toad and Mumen Rider try to stop it but fail, and Chain'n'toad is eaten. They are then saved by Atomic Samurai who kills the monster. Kamikaze and Bang talk about how many young heroes these days don't live to the essence of a "hero", and Bang states the Hero Association and their ranking system is partly to blame.

As Genos and Saitama watch TV, Saitama accidentally knocks Genos out trying to reach his potato chips and Genos wakes up with memory loss.

Saitama brings Genos to a tournament for the fighting video game "Wild Animals" due to the grand prize being five million yen. Several locations all over the country are being used as venues, and the battles take place online.

One stormy night, a monster emerges from a junkyard and begins making its way towards Smelly Lid Prison. Noticing its approach, Puri-Puri Prisoner breaks out of his cell and quickly defeats the monster. However, he damages the prison's electricity in the process, allowing a handful of inmates to escape.

Child Emperor, Zombieman, Superalloy Darkshine, and Metal Bat are dispatched to a forlorn, snowy mountain to hunt down a monster.

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?

Saitama is out shopping with Genos in tow when the pair come across G4, a demonic robot on a mission to eliminate King. Elsewhere, Sitch attempts to recruit various cutthroats, villains and assassins of the underworld to help the Hero Association.

Garou issues a declaration of war on the Hero Association through terrifying means, Hellish Blizzard gives Saitama an ultimatum, and Genos decides to eliminate his master’s stalker, Speed-o’-Sound Sonic.

Garou, out hunting heroes, can barely contain his excitement when he gets the chance to battle the S Class hero Tank-top Master. Meanwhile, Silverfang shutters his dojo to begin his own hunt of his former disciple.

Assigned to guard a VIP and his son, Metal Bat couldn't be less thrilled. But when the group is suddenly attacked all hell breaks loose. Meanwhile, Saitama has disguised himself as Charanko in order to enter the "Super Fight" martial arts tournament.

Monsters from the Monster Association launch simultaneous attacks on all the human cities, attempting to overwhelm the forces of the Hero Association. Meanwhile, the “Super Fight” martial arts tournament kicks off.

The Super Fight tournament kicks off while Genos takes on multiple monsters who have appeared in City C. Elsewhere, countless monsters are wreaking havoc in multiple areas, but the Hero Association is being overwhelmed by the sheer number of threats.

As the Super Fight tournament heads for its grand finale, the Class S heroes battle to hold back the tide of monster incursions. Meanwhile, Atomic Samurai is setting in motion a plan of his own.

Goketsu, the first Super Fight Tournament champion, arrives in the stadium. Having now become a monster, he’s aiming to force the Super Fight competitors to turn into monsters themselves. When several do, it’s up to Suiryu to take them on alone.

Just as Suiryu finds himself in the depths of despair, Saitama suddenly appears to help him out. But despite all Saitama’s strength, there’s still something bugging the hero that he just can’t seem to shake.

The Monster Association issues a challenge to the Hero Association, telling them they have only three days to rescue the hostage Waganma. Elsewhere, Garou tries to recover from his constant fighting, but things don’t quite go as planned.

Garou is at last surrounded by multiple heroes and forced to fight, even though he’s covered in wounds and nearly at his limit. The hero hunter still has his incredible ability to read opponents’ fighting styles, but there are surprises in store for all.

Silverfang and Bomb catch up with Garou while the hero hunter is battling Genos, starting a whirlwind battle that draws in more and more combatants as it marches towards its hair-raising climax!