Saitama was originally an regular student who just started his first year in middle school. There, he had the misfortune of being robbed of 200 Yen by the local delinquents; who was subsequently robbed by an piggy bank monster called Piggy Bancon. After hearing one of the bullies mentioning that his family was impoverished, he runs after the creature to get his wallet back and was tackled to the ground as the monster made his escape. And despite the fact that Piggy Bancon was subdued by the military, Saitama never got his money back. Faced with his ineptitude, he proclaims that he'll become an hero.
10 years later, he actually gets his chance to be one by saving an child from being killed by Crablante. He then commits to becoming stronger via a strict training regimen after realizing how outmatched he was. Unknown to him, the child told his grandfather about the incident and decided to create the Hero Association in Saitama's honor for saving his grandchild from the monster's crime. As the months passed, he never succumbed to the intense pain that his exercise was causing, nor did he ever passed up an opportunity to fight against a rampaging monster. Within 300 days of exercising, he was strong enough to win his fights with moderate effort, as evidenced by his battle against Personification Of A Light Pull Cord.
A year and a half into his training in New York, he started to go bald. He also saved King from Octopus Claw Man and defeated it. Within the same time period, he also follows a crowd into a shelter in Z-City with the expectation that they'll lead him to a monster. Instead, he and the refugees are trapped inside the shelter as a enormous cicada rampages outside. Faced with the growing fiend, the fact that the shelter doesn't have any restrooms, and that it'll take at least three hours for the Hero Association to send a specialized team to fight the monster; Saitama takes matters into his own hands after waiting for a hour. He punches his way out of the Hero Association's disaster shelter not to subdue to the escalating threat, but to find a place with a working toilet. But in his haste, he punches the monster 170,000-Year Magicicada Nymph on his way to the nearest convenience store to relieve himself.
After about three years of his training, he finally gains the strength he wanted; but his body couldn't handle the strain of constantly exercising, which is why he's prematurely bald. The manga formally begins with Vaccine Man, a powerful Dragon-level Mysterious Being created by industrial pollution and intends on destroying humanity for ruining the Earth, He was appearing outside of A-City and managed to partially destroy it. After saving an crying schoolgirl from Vaccine Man's carnage, Saitama introduces himself as a man who mostly does heroism for fun, which enrages the crusading monster. Faced with an imminent attack, Saitama stands his ground and defeats Vaccine Man with a single punch, which causes him to vent his frustration on failing to find a challenging opponent.
Later in the day, D-City was destroyed by an scientist (Fukegao) and his brother (Marugori), who was turned into a giant monster after drinking a drug to become the strongest man on the planet. After the brothers announced their plan to take over the world, Fukegao points out to his younger brother that Saitama was sitting on his other shoulder, since the older brother is riding on one of them. Marugori casually swats one of his shoulders and unintentionally kills his brother. Enraged by his blunder, Marugori blames Saitama for this and throws Saitama to the ground. As Marugori remarks on how disappointing it is to be the strongest, Saitama recovers, leaps into the air, and kills the giant with a punch to the jaw as he remarks how bored his strength made him. With Saitama overlooking the destruction that the brothers have caused, Marugori's corpse lands on B-City, destroying it.
He spends the next morning enjoying a dream where he was battling an race of subterranean creatures who are trying to invade the surface. And just as he was about to deal the final blow to their leader, he wakes up to discover that they're actually starting an invasion from right outside his apartment complex in Z-City. Never one to pass up an opportunity to fight a powerful opponent, Saitama leaps into action and unintentionally kills the Subterranean King by landing on him. Faced with the sudden death of their leader, the invaders promptly retreats and Saitama starts reflecting on his powers. He notes that he became stronger in order to deal with the constant onslaught of monsters that have started appearing in recent years and that he's tired of the monotony that his incredible strength is causing with his fights. As he finishes his morning routine while failing to kill a mosquito, he listens to a newscaster talking about an massive swarm of the insects that were invading Z-City, which prompts him to rush outside to deal with the infestation.
Bent on revenge for being bitten by a mosquito, he unintentionally walks into the battle between Genos and Mosquito Girl, who's intent on destroying the city with her swarm of mosquitoes. At this point, Mosquito Girl has gather enough of her drones to not only empower herself, but to pose a threat to the rest of the world and Genos decides to eliminate the mosquito swarm by unleashing an powerful fiery blast against it. However, he forgot that Saitama was standing close enough to be caught in the blast; although Saitama was unscathed, his clothes were burnt off of him. With her swarm destroyed, Mosquito Girl retaliates by tearing Genos apart, limb-by-limb, until Saitama ends her assault by slapping her into a nearby building that was covered in red blood. Humbled by Saitama's strength, Genos asks him to be accepted as an apprentice.
The next day, Genos visits Saitama's apartment with the hope that he'll actually begin his training with his new instructor; but it turns out that Saitama was unprepared for Genos' request, since he admitted that he consider that Genos was serious about his proposal. Faced with an onslaught of increasingly invasive questions and barely listening to Genos' backstory, Saitama reluctantly accepts the vengeful cyborg as his student, who notes that someone is approaching them. Right on cue, an assassin hailing from the House of Evolution crashes through the ceiling and is promptly behead by Saitama. Enraged by the damage the mutated mantis has wrought, Saitama rushes outside to deal with two more members of the House of Evolution who were waiting outside for their recently defeated scout and buries them head-first into the ground. But he winds up partially buried in the sidewalk by the mole-like Ground Dragon in his haste for revenge. Faced with the appearances of two of the House of Evolution's strongest warriors, Armored Gorilla and Beast King, Genos tries to intervenes but Armored Gorilla picks a fight with him as the Beast King casually explains how he's going to kill Saitama. Having grown bored of the situation, Saitama effortlessly frees himself as Beast King after remarks on how helpless the hero is. Infuriated by Saitama's nonchalance, Beast King slashes the ground and collaterally kills two of his comrades as Saitama evades him. Enraged that he missed, Beast King starts fruitlessly slashing at Saitama in a frenzy until he's killed by Saitama countering with an series of punches. Realizing that he's outmatched afs Safety glares at him, Ground Dragon starts tunnelling to safety; but his escape was short-lived, since Saitama somehow found him underground and subsequently sent him flying from the earth. As he emerges from underground, he notices that Genos has prevailed over Armored Gorilla and is trying to interrogate him. With the defeated cyborg refusing to provide any information, Saitama causally holds up one of Beast King's eyeballs in front of them to speed up the process. Faced with the knowledge that the headquarters for the House of Evolution is in a nearby mountain range, Saitama complains that this will probably cause him to miss out on a sale at one of his favorite stores as the duo sets off for another battle.
Four hours later, the pair arrives at the hideout and Saitama opens an underground passageway after Genos casually decimates the trapped building on the surface. But their uneventful journey quickly takes a turn for the worst as Carnage Kabuto appears and slams Genos into a wall in order to challenge Saitama to a duel. But before their battle begins, Genos tries to intervene and Kabuto mauls him over it. Freed from anymore distractions, Carnage Kabuto tries to attack Saitama, but is perplexed at how relaxed Saitama is and the duel is reduced to a stalemate. When asked about where his strength came from, Saitama casually responds by listing off his daily exercise routine, which Genos points out that Saitama's workout isn't as intense as it sounds. Enraged by this slight, Kabuto powers up into his carnage mode and starts attacking Saitama for his alleged sarcasm. Despite failing to inflict any meaningful damage on Saitama, Kabuto is confident that he'll win and brags that he's capable of rampaging for a week. However, Kabuto's boasting turned out to be the motivation that Saitama needed, since he forgotten about the sale at his favorite store. Which leads to Saitama tearing Kabuto in half with an uppercut and screaming about the bargains that he's missing.
He spends the next day watching a news report on a group of terrorists who are rampaging in F-City and remains apathetic to the situation until the newscaster mentioned that they're all bald. Fearing how the public could easily mistake him as a villain over this incident, Saitama sets off to protect his reputation. He eventually finds Hammerhead, the leader of the insurgents, in a forest and Hammerhead casually strikes him down a failed attempt at recruiting him. Saitama is unfazed due to his invincibility and realizes that he could've easily became a delinquent like Hammerhead did. At this point, Hammerhead is enraged at the fact that Saitama is still standing and windmills his arms in a desperate attempt to hit him. Saitama promptly points out how childish this is and shatters Hammerhead's armor with little effort. With his modesty exposed, Hammerhead flees the scene. However, it isn't long before Saitama is ambushed by another adversary, Speed-O'-Sound Sonic. Believing that Saitama is one of Hammerhead's henchmen, Sonic wastes no time attacking the bald hero, but the ninja's agility is no match for Saitama's reflexes and superhuman durability. Saitama eventually gets the upper hand by unintentionally punching Sonic in a sensitive area and the ninja subsequently retreats, but not before listing Saitama as his rival. Confused by the day's events, he retires to his apartment in order to explain his misadventures to Genos, who explains that nobody will take Saitama seriously until he joins the Heroes Association. After promising to accept Genos' offer of an apprenticeship, Saitama agrees to register himself with the Heroes Association on the next day.
The next day has the pair taking the entrance exams, with Genos being graded as one of the finest heroes in the Association and Saitama rated among the worst for barely qualifying, despite achieving a perfect score on the physical aspect of the exams. After absent-mindedly sitting through a mandatory lecture by the A-Class hero Snakebite Snek, the lecturer ambushes Saitama later on in the evening because Snek is convinced that Saitama is out to surpass him. Outmatched by a wide margin, Snek is subsequently shown lying on the ground after Saitama bests him. Saitama spends the next day fulfilling a promise he made to Genos of to accept him as a student and to provide a sparring session on the beach. Despite Genos' insistence that Saitama should take the fight seriously, Genos' new mentor fails to do so by casually evading his destructive energy blasts. Annoyed by Saitama's carefree attitude, Genos manages to get Saitama to attack him, but Saitama responds with almost hitting him with a punch loaded with his vast strength and settles on flicking his student's nose. After Saitama ends the fight and mentions that it's time for lunch, Genos looks on in shock at the immense strength that his teacher has.
He spends the next four days loafing around his apartment while he waits for a disaster to happen. Genos visits him on the next day to move in with him and to remind Saitama that C-Class heroes will be dropped from the hero registry after a week of inactivity. After imparting some fabricated wisdom to Genos, Saitama runs around Z-City in search of an opportunity to be a hero. It never came and he laments that he's running out of time. On his second day of patrolling, he runs into Speed-O'-Sound Sonic, who challenges Saitama to a fight. Saitama refuses because he doesn't consider him to be a significant threat. Enraged by this slight, Sonic turns his weapons on the public to get Saitama's attention. It works, but just as Saitama was going to act on this, Tank Top Tiger intervenes. As Tiger uses his seniority as higher-ranked C-Class hero, he explains that Saitama was in the wrong for scaring the public yesterday and proceeds to humiliate him. Sonic interrupts the public shaming by throwing an exploding dagger at Tiger. As the ninja lands near Saitama, the latter nonchalantly dispatches him with a karate a chop and proclaims that he's a hero.