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 tells him to meet him in the teacher's 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, gets 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 up to it, but gets knocked out for at least an hour. He then shows up at the teacher's lounge, beaten and his uniform torn apart, and gets shouted at. After leaving the teacher's lounge, he starts questioning his future. The scene then jumps forward in time to 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. He is rewarded with a year worth supply of candy. Afterwards, Saitama thinks of how he has taken down over 10 mysterious beings since he became a hero for a hobby, how he is not doing it for the gratitude, and how he has been pushing his body to the limits.
On his 300th day of training, Saitama starts to feel an unbearable stinging pain. As he does his daily jogging, he starts moving 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. Saitama shows 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 about how strong it is, and destroying a nearby vehicle with rapid punches. Saitama approaches the monster, but is unsure of his success because of all the pain. The mysterious being is revealed to be the Tiger level monster Personification Of A Light Pull Cord, who talks about 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 sets his sights on continuing, as he did not come so far for nothing. Then, right 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.
During Saitama's first year and a half of training, Saitama is at Z-City's Shelter Nr. 7 hiding from a mysterious being. Saitama then realizes the heroes that were sent to deal with the monster were all defeated, so he attempts to escape, but the other civilians stop him, thinking he has become suicidal. Saitama then notices he has to use the bathroom but cannot find it. He asks a female staff member of the Hero Association where it is, to which she replies there is none, which Saitama states is a huge flaw.
One hour later, Saitama is struggling with his filled bladder. A person on the phone says that the mysterious being's threat level has just risen to Demon, and that they now must wait for the newly formed "subjugation squad", which will arrive in 3 hours. Upon hearing this, Saitama moves rashly and proceeds to punch a hole in a wall, saying that he is going to the convenience store, much to the crowd's surprise. Saitama then runs towards the convenience store, and is interrupted by the Magicicada Nymph, who is swiftly killed by the rushing Saitama.
After Tatsumaki leaves the fight against Demonic Fan with Fubuki, Saitama shows up and notices that 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 Demonic Fan. He patronizes the coward hero, his words reaching the hero. Inspired by Saitama, the hero left the group and started to train himself in order to get stronger.
Saitama tries to withdraw money for drinks, but doesn't have enough money. When Bull-Bull is about to rob the bank, Saitama stops him. When Saitama returns home, he is surprised to see Genos bought a lot of the drink he wanted. He later makes Mumen Rider take the drinks.
A man is standing on the top of a high building, claiming he is about to jump off because his life fell apart. Saitama is sitting on the building right next to the man, eating his lunch. The man thinks Saitama is gonna try and stop him from jumping. As the man is trying to tell Saitama about the harshness of life, Saitama drops a piece of his salmon that he was saving for last, and runs down to catch it and then jumps back up to the roof of the building at immense speed. The man, in shock over seeing Saitama's speed, slips and falls, but Saitama catches him with his chopsticks, accidentally dropping his salmon instead. Saitama then encourages the man, who is grateful, but storms out angrily. A monster then appears, and Saitama prepares to fight it.
Saitama is bothered by a rock in his boot, one that he got from the moon. He then asks Genos how much is a moon rock worth, who tells him one gram is worth 600,000¥. Saitama then hurries to go sell it, but accidentally drops it in a nearby parking lot. Saitama and Genos spend all night, unsuccessfully looking for the rock.policemen looking over for any suspicious activity while on patrol. After some discussion, they take him in for questioning. Through further investigation into his background, the two police officers 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 to him by Chief Kuma. As they Are still conversing, the real culprit behind the assaults appears; a Demon level monster threat named Surprise-Attack Plum. It starts attacking their establishment. 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 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-handily 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 "Water Stream 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 assault him. Charanko tries to call for help, but Saitama and Genos are still cooking the hotpot and are not aware of what is happening. After Charanko is kicked back and almost lands on the hotpot, Saitama catches him using his chopsticks. 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 through 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.
Saitama briefly appears to take care of a monster at the business district.
King is showing Saitama a new anime series about a superhero who uses baked potatoes as weapons, telling him that it is very popular, much to Saitama's uncertainty. Genos arrives from the cleaners with his hero uniform, much to Saitama's excitement, while King is questioning the matter. Saitama says he entered a hero costume contest and asks King to join him, since there is prize money, but King declines.
At the center, Saitama and Genos notice a crying boy, wearing only a pair of shorts. The boy informs them that he is always bullied because his father makes him wear tacky clothes, and that the bullies took his clothes earlier, while his ticket was in his jacket. Saitama gives the boy his hero uniform and tells him to go home. Genos wonders how Saitama will win the contest without his costume, only for Saitama to have backup clothes underneath, which he won from Mouse Sushi by filling out a stamp card.
The contest begins, and Saitama appears last. While walking down the catwalk, many comments were made with Saitama informing everyone he is a B-Class hero and will show them his special power - wiggling his ears. The head judge Torajirou criticizes Saitama for his lack of fashion sense and scorns him as a horrible person, based on his clothes, not allowing Saitama to speak. Just as the judges were about to give him his score, Game-Berus appears at the stadium attacking the audience. The heroes that were there are easily defeated, and just as the monster was hurling towards the judges, Saitama steps in and kills it with one punch. This action gained favor in Saitama from the audience, nevertheless he received a total score of 8. As he was heading home, he meets the boy from earlier and the boy returns Saitama's hero uniform, thanking him.
Saitama briefly appears, killing monsters that Tatsumaki wanted to.
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 to a fight. He uses his new weapons against Saitama, but it is ineffective and Sonic leaves 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 is King's turn to play, 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.
When asked about his holiday plans, Saitama says that he is probably just going to eat some oden.
King is visiting Saitama and showing him a new series called ZOO-MEN. Genos arrives and King asks what Genos' opinion on team based heroes is, which the former completely disregards along with the show. Genos then hands Saitama a letter from the Hero Association.
At 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 offers himself to handle the monster if the plan goes awry and things get out of hand, which offends 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 is 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 tries to warn the heroes, but they leave him behind.
In the forest, Scaledon falls into the trap, after it was lured by the scent. The monster tries to break free, only to release another trap. Smell Master attempts to use a smell to paralyze it, but is struck by the monster's attack. The mysterious being 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 realizes that fiction and reality are different and working alone is for the best.
At home, King asks Saitama about his mission, while they 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.
Saitama was supposed to meet King inside a restaurant, but was late. He appears at the end of the omake, defeating some assassins that try to kill King.
After Saitama saves a street food vendor from a monster, the vendor thanks Saitama and asks about disaster levels. Saitama simply does not know and instead asks for a baked potato.
Saitama later passes by and Withered Sprout and the monster uses a sneak-attack and freezes Saitama. Saitama easily breaks his glaciers, but Withered Sprout attacks Saitama again only to be killed by Saitama's punch, for freezing his potato.
Saitama later meets Genos, who is burning a pile of monster corpses and explains he got damaged by a Demon-level monster, but Saitama was simply trying to roast his potato.
Vaccine Man attacks A-City as the Hero Association's staff rush to get in contact with as many heroes as possible. Saitama kills the monster before any S-Class heroes arrive. Shortly afterwards, King stumbles onto Vaccine Man's corpse when picking up his video game, Moe Moe Chronicle 2 from the floor, and is mistaken as the monster's killer.
Saitama and Genos are at the beach when Saitama asks Genos if he can swim. Genos explains that Dr. Kuseno made him some parts for swimming and leaves to prepare them. while relaxing in the ocean, Saitama wonders where Genos is and calls out his name. Suddenly, a periscope lifts Saitama, revealing Genos as a submarine with Saitama yelling to get a hold of himself.
Saitama is walking around Z-City reading a manga, thinking about the manga hero's journey, finishing moves, and hypothetical monsters. While wandering, Saitama encounters three monsters called the The Tempest Brothers. The monsters are annoyed by Saitama's obliviousness to their presence, but after Saitama easily kills one of them, the other two run back home.
During Saitama's 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 Saitama's 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 comes to visit and notes that he didn't pay the 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 boss' stooges comes to claim the store's deed. As Saitama gives counsel to the clothes-shop owner not to 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 a mysterious being, Fish of Darkness. Saitama and the boss have a brief fight, with the latter having the upper hand, until he mentions the reward on his head, which fires Saitama up. Saitama returns to the Landlady, saying he's called the police and that 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. The 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 had made for Saitama, which turns out to be Saitama's hero outfit. In the present, after hearing Saitama's story, Genos impulsively thinks Saitama should get a better suit, and Saitama asks him if he was really listening when he explained his suit was also a memento.
Genos stalks Saitama in order to find out the secret to the Saitama's strength as he goes about living his daily life, at one point even going so far as to stealing one of his fries, a long french fry, the only one Saitama liked.
Saitama encounters the B-Class criminal Men's Esthetician Man, who recalls Saitama as someone who has beaten him before. Saitama on the other hand, does not remember Men's Esthetician Man. However, he is convinced that he was the stalker who stole his french fry. We skip ahead to seeing a hole in the wall in the shape of Men's Esthetician Man and Saitama telling him to next time just approach him, rather than stealing his food. Saitama is later seen going back home and complaining on not getting any recognition.
Saitama and Genos are in a communal bath, discussing the various sounds that would be fitting while taking a hot bath. Saitama then tells Genos to go home, gives him his room keys and warns him not to lose them. Genos waits for Saitama, while Saitama stops by a food store and decides 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 is over. Genos arrives 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.
Saitama is searching for food and happens to run into a wild bear who was attacking a hunter. At first, Saitama was eyeing a baby boar, but it ran away, leaving Saitama to take the bear instead. At his apartment, Saitama and Genos have hotpot with bear meat.
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. Saitama is challenged to a game of rock, paper, scissors by Bang and loses several times. When he finally wins he almost kills Bang by accident.
Saitama is seen waiting for Genos at home, commenting about the latter being very late.
After the Dark Matter Thieves' failed invasion, the heroes that participated are invited to a hot spring resort, when Zombieman is suddenly found murdered on the floor. Child Emperor gathers some of the evidence, one of them, being the weapon with which Zombieman was stabbed, Atomic Samurai's sword. Child Emperor attempts to interrogate and eliminate the suspects one by one, including Saitama, who was suspected, as he asked Zombieman if he truly is immortal, for which Genos threatens Child Emperor of even thinking Saitama was a suspect, informing him that if Saitama were the murderer, he wouldn't have needed to use the sword.
It is later revealed that Zombieman was just asleep, and that Tatsumaki was the one who stabbed him while intoxicated. While walking home, Saitama is wondering why Tatsumaki would stab Zombieman. Genos knew it was Saitama's fault, but doesn't tell him. It is revealed that Saitama told Tatsumaki that she is a child, and shouldn't drink adult sake. Feeling enraged and humiliated, Tatsumaki started drinking as much as she could from Saitama's bottle of sake.
Saitama meets a group of villains in a public restroom who mistake him for their ally. They introduce themselves to Saitama, but when Saitama reveals that he is a hero, they attempt to defeat him. Their attacks misfire and they end up getting saved by Saitama, and start contemplating about how weak their abilities are compared to the likes of Tatsumaki and Volten, who use their respective abilities to earn a living. They have a change of heart, but the group is then attacked by Dirt Earthworm and saved by Saitama again, who sends the monster flying. Genos arrives at the scene after following the evacuation warning only to find Saitama there alone with The Mediocre Ability Users, who he thinks are troubling his master. They quickly lie about how they just aspire to be heroes and wanted to spar with Saitama, who believes them and decides to train them.
Saitama and Genos are watching TV, when Saitama accidentally knocks Genos out, causing Genos to lose his memory.
Saitama and Genos take part in a video-game competition.
Genos asks Saitama to test the VGS system. Saitama easily defeats several monsters and a ghost of Genos. Saitama gets upset that he punched holes in his apartment's walls while doing the simulator. He is called by Genos do to the simulator again and defeats a ghost of his past self. Saitama then tell Genos to fix the walls.
Sonic, Becoming Saitama
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 while fighting, claiming food as 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 Energiser. 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 Energiser. Saitama questions Sonic's attire and asks if he wants to be a hero. Sonic denies, and claims 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.
Atomic Samurai and Tatsumaki stumble upon the corpse of Energiser, with Saitama nearby. Atomic Samurai asks what happened and why Saitama is there, but Saitama responds with asking who Atomic Samurai is. Incensed, Atomic Samurai 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 him to drag his name through mud.
Saitama and Genos are out on patrol when Saitama gets hungry. Having forgetting his wallet at home, Saitama takes on the jumbo pot sticker challenge to hopefully get free food if he wins. Unfortunately the hero failed and rushes home get his wallet to pay 5000 yen in fees. While they head home, they run into Charanko. Saitama forgets who he is angering Charanko but upon hearing his desire to become a hero offers advice to him. Saitama requests for Charnko's first hero act to lend Saitama 5000 yen to which he vehemently denies.
Saitama is a playable character in the video game, One Punch Man: A Hero Nobody Knows. He retains his ability to defeat his opponents within one hit and invulnerability. If his character is selected, however, a special timer is set before he becomes available to fight. Once the timer runs out, he arrives on the scene and is fully playable.
An alternate version of Saitama known as "Dream Saitama" is also playable. He wears his pajamas from his dream fight against the Subterraneans. In this form, he is available at the very start of the battle and takes damage like a normal character.
Saitama is one of the various characters able to be summoned. He comes equipped with his standard hero gear and has devastating physical prowess.