After Tatsumaki leaves the fight against the Demonic Fan with Fubuki, Saitama shows up and notices that the Demonic Fan is not dead, but instead reforming itself. The young member of the Blizzard Group attempts to stop Saitama from going to attack the Mysterious Being, but Saitama goes anyways and easily destroys the Demonic Fan. He patronizes the coward hero and his words reached the latter. Inspired by Saitama, the hero left the group and started to train himself in order to get stronger.Chief Kuma. However, as they were still conversing, the real culprit behind the assault of officers appears; a demon level monster threat named Surprise-Attack Plum began attacking their establishment after shedding his delivery man disguise stating his hatred and assault on the Law was out of revenge for busting him earlier in his life. With the disaster at hand, Chief Kuma had to be dragged away from the questioning room to observe the threat. Chief Kuma calls the Hero Association out of desperation. The derogatory second chief of operations for the Hero Association, McCoy, requested his opposing heckler to cast aside his and the face of the police. Saitama quickly snatched away Kuma's cellphone, crushing it before he could voice his dejected humility, clarifying that neither heroes nor cops do their job because people say "I beg you" and that the man on the other end of Kuma's phone wasn't any kind of hero.
Looking to pay the good police chief back for the free food, Saitama offered to handle things on behalf of the men and women of the force and single-handedly took out the monster. Outside of the police station, countless bystanders, the local news and a horde of heroes from Z City Hero Association HQ sent by the duplicitous operator McCoy, who leaked the information to the media and sent out every available upper-rank hero in order to further disgrace the police, appear. McCoy's plan backfires when a lone police officer steps out of the building carrying out the downed creature on his back, much to everyone's shock and awe as well as McCoy's chagrin. All the news channel sponsors present at the scene bustle their way into a police blockade, desperate to know who the mystery officer was that took out the monster attacking them at their office. Before anybody could thank him, Saitama had already left the scene, content with having helped the police save face.
Saitama is looking for a monster in the K-City Nature Park when he bumps into a fisherman. Saitama and the fisherman have a short conversation about the fisherman's philosophy, allowing an opponent to grow and have another matchup. The fisherman then manages to catch the "King of the Lake", but it is eaten by Lord Great White, the last survivor of the Seafolk invasion. Afterwards, Saitama quickly takes care of Lord Great White and parts ways with the fisherman. On his way, Saitama thinks about the fisherman's statements and tries applying it to his hero career, remarking that unlike the fisherman, he does not have a good competitor. Suddenly, Speed-o'-Sound Sonic appears before Saitama and challenges Saitama into a fight. He uses his new weapons against Saitama, but it useless and Sonic must leave the match defeated once again.
As part of her ongoing efforts to recruit Saitama into her group, Fubuki challenges Saitama and his group to another match with the losers having to do whatever the winner says. The next day, Saitama alongside "his group" consisting of Genos, Bang and King, all arrive at the Blizzard groups hideout and Saitama signs their contract without even reading it.
Quickly caught in a video game contest, Saitama, Genos and Bang all lose in rapid succession to the Blizzard group. However, when it's King's turn he dominates the games and quickly beats all thirty present Blizzard group members. Later the group encounters Child Emperor with his new invention Okame-Chan and Jumping Spider who was rampaging in the area.
King and Saitama both get ratings of "unknown" from the device; according to Child Emperor the device has a certain limit if a person is too strong or weak. When Fubuki gets rated by the device, however, it registers her as 19 when the average C-Class is 100. After Fubuki falls into a depression Saitama snaps her out of it by telling her to stop caring so much about what others think of her.
With Fubuki snapped out of her depression she easily destroys the Parasites produced by Jumping Spider after he died. With the battle over Saitama reminds her that she owes him a favor, ultimately treating Saitama's group to a dinner at Restaurant Mammoth.
He defeats a monster and buys a potato. On his way home, he encounters Withered Sprout, he defeats him and inadvertently saves the Surly Brothers. He is later at his apartment, talking to Genos and tries to unfreeze a potato that was frozen, with a monster's burning corpse.
OVAEditZ-City. During one of his errands, he tore his tracksuit and visited a shop owner, who was one of the few people who supported his training. After leaving his suit, the Draper warns Saitama to be careful, since criminal activity increased in the area, something Saitama himself noticed. At his old apartment, the Landlady came to visit and noted that he didn't pay rent in three months. She proceeds to warn Saitama that she will kick him out. As Saitama was picking up his tracksuit, one of the gang bosses stooges came to claim the store's deed. As Saitama gives counsel to the clothes-shop owner to not give up, he points him the direction of the gang's base which turns out to be his apartment complex. Saitama goes door to door meeting various criminals who work under the boss, but has no real interest until he hears them each mention a reward that's on their heads and soundly beats them. The boss is the only one left, after Saitama's tells about no need for overcharging, the boss loses it and transforms into the Fish of Darkness. Saitama and the boss have a brief fight, until the boss mentions the reward on his head. Saitama returns to the landlady saying he's called the police and the reward should pay for his rent, but the Landlady is revealed to be crooked, stating those tenants were her main source of revenue. She then throws Saitama out into the street with his belongings in boxes. Saitama later returns to the shop owner to explain what happened, shop owner then states that he was getting too old and has decided to close the shop anyway but not after finishing a masterpiece that he made for Saitama, which turns out to be Saitama's hero outfit. Now in the present after hearing his story, Genos impulsively thinks Saitama should get a better suit, but Saitama asks him if he was really listening when he explained his suit was also a memento.
Saitama was searching for food and happened to show up when a wild bear was attacking a hunter. At first, he was eyeing a baby boar, but it ran away, leaving Saitama to take the bear instead. At his apartment Saitama and Genos are having hot pot with bear meat, with Genos taking notes about it.
Saitama is seen waiting for Genos at home, commenting about the latter being very late.
Saitama caused the events of the Ova after he denied Tatsumaki a drink.