A 12 year old Saitama is dozing off during his middle school opening ceremony. Later, Saitama is relaxing in his bed watching a news report about an escaped Mysterious Being. Soon after, a teacher berates Saitama for not doing his homework, and telling him to meet him in the teachers lounge after school. After school ended, Saitama is accosted by the two delinquents known as Ripped Uniforms. The two drag Saitama out to the school yard and ask him for his money. Saitama refuses and then prepares to fight them, but gets easily beaten up and his money, 200 Yen, stolen.
Piggy Bancon shows up and takes all the money the two delinquents had, including Saitama's money, then runs away. Saitama gets up and chases after the monster. He catches it but gets knocked out for at least an hour. He then shows up at the teachers lounge, beaten and uniform torn apart, and gets shouted at. After leaving the teacher lounge, he started to question his future, if he will be able to live properly. The scene then jumps forward in time to a Saitama carrying a child on his shoulders on a snowy hill having just defeated a snowman monster. The child then asks if Saitama can beat up another snow monster, this one being significantly larger. At this point Saitama realizes he forgot to take out the trash.
During Saitama's training days where he was aiming to become a hero and had hair, Saitama saved an old lady from a mysterious being, and is rewarded with a year worth supply of candy. Afterwards he thinks of how he has taken down over 10 mysterious beings since he became a hero for a hobby, and how he is not doing it for the gratitude but it does help him, and even how he has been pushing his body to the limits.
At his 300th day of training, Saitama starts to feel an unbearable stinging pain. As he does his daily jogging, he is shown to move at high speed, and thinks of how he is gradually getting used to the training, and how it even gave him a boost in his physical fitness, showing what he can do by saving a child from a speeding car, crushing its hood in the process. Saitama disciplines the child about road safety, while people around are awing at his amazing feat.
Continuing his thought of his growth, Saitama claims that he is able to move of how he thought superheroes would but there is still one flaw to his training, the stinging pain, which became more frequent. A Mysterious Being then appears, boasting of how strong it is, and destroying a nearby vehicle, with rapid punches. Saitama approaches the being, but is unsure of his success, because of the pain. The mysterious being is revealed to be the Tiger level monster Personification Of A Light Pull Cord, who talks of how he has changed and claims to be a "God" level threat. Personification Of A Light Pull Cord then notices Saitama coming from behind to attack him, and as Saitama is distracted by his stinging pain, the monster punches him through a truck and into a wall. Saitama then thinks he is at his limits, but then is determined to continue as he has not came so far for nothing. Then before Saitama is even capable of returning to the fight, Personification Of A Light Pull Cord attacks him with his Pull Cord Rush. Saitama then appears to be down, and a tooth falls out of his mouth. Saitama then recuperates, much to Personification Of A Light Pull Cord's surprise. Feeling much better now that his decaying tooth is out (which was the origin of his stinging pain), Saitama beats Personification Of A Light Pull Cord.
Back to the original story line time. Saitama is seen leaving the bathroom, after seemingly brushing his teeth, and thinks of how maintaining healthy teeth prevents tooth decay.
A New Wind BlowsEdit
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.
What Can't Be BoughtEdit
Pork Cutlet BowlEditAs Saitama is out on a walk he's pulled over by two policemen looking over for any suspicious activity while on patrol and after some discussion, they take him in for questioning. Through further investigation into his background, the two lawmen come to the conclusion that Saitama may be the culprit behind a string of grisly assaults that have been occurring around their precinct. While admitting that an unpopular hero is no different than a part-timer, Saitama wonders what the police force's bone to pick with the Hero Association was before enjoying some pork cutlet offered by Chief Kuma. But as they were still conversing the real culprit behind the assault of officer appears; a demon level monster threat named Surprise-Attack Plum began attacking their establishment. With the disaster at hand, Chief Kuma had to be dragged away from the questioning room to observe the threat. Chef Kuma calls the Hero Association out of desperation. The derogatory second chief of operations for the Hero Association McCoy desired 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 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.
Saitama offered to handle things on behalf of the men and women of the force and single handedly took out the monster. Outside the police station, where countless bystanders, the local news and a horde of heroes from Z-City Hero Association Headquarters 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. 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.
After being called over to Bang's dojo, Saitama learns that Bang received some lobsters from his brother and needs someone to help him eat them. Genos complains about them being called out for something insignificant, but Saitama tells him to stop, saying that they're only helping a lonely old man with his food problem. After Charanko arrives with the vegetables, they start preparing the hotpot while talking about a recent dojo hunter that has been attacking other dojos. Saitama notices that there isn't any cabbage, interrupting the conversation, which annoys Charanko. After some quarrel, between Genos and Charanko, the latter challenges Genos to a match, but Bang tries to calm down the situation by saying that he will go buy the cabbage and asks Charanko to get along with the other two.
After preparing the hotpot, they hear a loud voice calling a challenge. Inside the dojo enter a large group of martial artists, claiming to be from the "Harsh Path Style" and asking for Bang, master of the "Flowing Water, Rock Smashing Fist", so that they may crush the dojo. After Charanko chastises them, saying that this dojo does not tolerate dojo hunting, the martial artists challenge him instead and start attacking him. Charanko tries to call for help, but Saitama and Genos are still cooking the hotpot and don't notice. After Charanko is kicked back and almost lands on the hotpot, Saitama catches him using his chopsticks, holding him up. One of the martial artists sees the lobster and moves to crush it saying that it is a luxury not fitting of a martial artist, at which point Genos launches across the room, saving it. When the group tries to attack, Genos knocks one of them threw the wall with a single push, leaving a hole in the wall. Saitama praises Genos, saying that while it might not be their job to subdue dojo hunters, these ones had crossed the line, at which point they try to attack him. Saitama quickly grabs and throws all of them using his chopsticks, leaving several more holes in the wall. After Bang arrives, he seems rather upset about the condition of the dojo and asks the last remaining member of the group if they had done it. When the guy tries to say it was Saitama and Genos, Bang angrily says that they are the ones picking a fight here. After the group leaves, Saitama is seen enthusiastically enjoying the hotpot.
Tatsumaki's Day OffEdit
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 Group's 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 20 when the average C-Class is 100. After Fubuki falls into 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.
 King is visiting Saitama and showing him a new series called ZOO-MEN. Genos arrives and King asks Genos' opinion on team based heroes, which the former completely disregards that ideology and the show. Genos then hands Saitama a letter from the Hero Association.
At the Z-City's Hero Association Branch, the staff is explaining to the B-Class heroes; Wild Horn, Pink Hornet, Trap Tengu and Smell Master their mission to capture a wolf level monster. Wild Horn is assigned as the leader. The location of the monster is in the forest between Z-City and Y-City. Wild Horn gives orders to the other heroes to catch the monster, but only entrusts Saitama with a map. Saitama offered himself to handle the monster if the plan goes awry and things get out of hand, which offended Wild Horn and prompts him to demonstrate his weapon. In the forest the heroes prepare traps and notice scratch marks on the trees, while Saitama was distracted by a vending machine. This annoys Wild Horn who lectures Saitama on his lack of heroism. As the heroes go ahead, Saitama notices a large hole in the vending machine and footprints. Saitama tried to warn the heroes, but they left him behind.
In the forest, Scaledon fell into the trap, after it was lured by the scent. The monster tried to break free only to release another trap. Smell Master tried to use a smell to paralyze it, but was struck by the monsters attack. The creature breaks free and quickly devours the heroes. Saitama appears shortly after by following the sound, complaining he was left behind and demands for the monster to spit them out. The monster promptly eats Saitama as well, causing the hero to come to the conclusion that bringing in the monster alive isn't an option. He meets the eaten heroes in the stomach and punches the monster, killing it, stating they share the responsibility. Thinking to himself, he realized that fiction and reality is different and working alone is for the best.
At home, King asks Saitama about his mission, while both play a video game. Saitama expresses his belief about teamwork and its shortcomings. While Saitama's video game character gets poisoned, he asks King for help, only for the latter not to provide any after what Saitama said earlier.
King's Weekend-like WeekdayEdit
Saitama was supposed to meet King inside the restaurant. He appears at the end of the Omake, defeating the assassins that tried to kill King.
Road To HeroEditDuring Saitama hero training when he still had hair and his costume was a track suit, he continually defeated numerous criminals in Z-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.
A Shadow That Snuck Up Too CloseEdit
The Pupil Who Is an Extremely Poor TalkerEditSaitama and Genos were in a communal bath, discussing the various sounds that would be fitting while taking a hot bath. After the conversation Saitama tells Genos to go home, he gives his room keys and warns him not to lose it. Genos waited for Saitama, while going home Saitama noticed a food store and decided to cook hotpot for dinner. He needed some ingredients so he took out leaflets and saw that there are sales for high-quality chicken thighs in a supermarket, Genos suggested that if they split they could take the ingredients before it was over. Genos arrived home and tells the story of what happened, but Saitama had a spare key and didn't worry about the lack of meat. Genos then surprises Saitama with an expensive meat he bought at a department as an alternative and both eat the cooked dinner.
The Ninja Who Is Too ComplicatedEdit
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.
Bang, Who Is Too OverbearingEdit
In this special episode Bang tries to recruit Saitama by talking to him about how he got his powers. Then after a few minutes bang finds Saitama and Genos in a truck crash and asks them to have hotpot with them.
The Sisters Who Have Too Many Things HappeningEdit
Saitama is seen waiting for Genos at home, commenting about the latter being very late.
The Murder Case That Is Too ImpossibleEdit
Sonic, Becoming SaitamaEdit
Speed-o'-Sound Sonic challenges Saitama to another match and Sonic experiences his 11th loss although Saitama thinks it's the fourth. Sonic is outraged over Saitama forgetting their epic battles but Saitama is always eating when he attacks and he place food a higher priority than Sonic. Saitama eats his slightly burnt cup noodles and asks that Sonic times his attacks during the three minute wait for his cup noodles to cook. Sometime later, Saitama and Genos split to find a monster.
Saitama encounters Sonic battling Energira. Despite Sonic's skill, Sonic loses the battle and almost dies if it weren't for the timely arrival of Saitama. Together they take down Energira. Saitama's questions Sonic' attire and asks if he wants to be a hero. Sonic says he's not and that he knows Saitama's weakness. Once more, Sonic declares that he will end Saitama's life. Sonic then leaves with a final message saying that Saitama will soon know his weakness.
Kamikaze and Tatsumaki stumbles upon the corpse of Energira with Saitama nearby. Kamikaze asks what happened and why Saitama is there but Saitama responds with asking who Kamikaze is. Incensed, Kamikaze points out that it is the second time they met today and asks if he killed the monster as it was a powerful mysterious being. Tatsumaki pesters Saitama over killing the monster meant for them and insults Saitama over his bald head for revenge. Saitama then realizes that the weakness might be that Sonic pretended to be Saitama to drag his name through mud.