A destructive creature called Vaccine Man attacks a city and threatens a young girl, only to be eliminated instantly by Saitama, a bald hero whose overwhelming power has made victory feel meaningless. Three years earlier, Saitama was an unemployed man who saved a cleft-chinned child from the crab monster Crablante. Surviving that encounter inspired him to train until he could defeat any enemy with one punch. In the present, he destroys a giant created by the Brain and Brawn Brothers, but the effortless result leaves him as bored as before. That night, Saitama dreams that subterranean invaders overwhelm the surface world. For the first time in years, their strength forces him to struggle, and the excitement of a genuine battle restores his sense of purpose. He awakens to find actual Subterraneans emerging outside. Unfortunately, their king falls to a single punch, and the remaining creatures surrender immediately. Saitama is left standing in silence, frustrated that even the enemy from his most thrilling dream cannot challenge him in reality. The episode establishes both his unmatched ability and the emotional emptiness created by having already surpassed every conventional obstacle.
Episode 2
あらすじ
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Genos, a young cyborg searching for the machine that destroyed his hometown, confronts Mosquito Girl and the vast swarm she controls. His incineration attacks destroy thousands of insects, but she absorbs enough blood to transform and overwhelm him. Preparing to self-destruct rather than let her continue, Genos is interrupted when Saitama wanders into the battle and kills her with a casual slap. Fascinated by such power, Genos follows Saitama home and delivers an exhaustively detailed account of his past. Saitama barely listens but eventually accepts him as a disciple, mainly to end the monologue. Their new arrangement is tested almost immediately when modified beasts from the House of Evolution attack. Saitama defeats Beast King and the other creatures with little effort, while Genos battles Armored Gorilla. After witnessing his allies' destruction, the gorilla abandons his artificial manner of speech and cooperates. He reveals that the scientist Dr. Genus created the organization and now wants Saitama captured for study. Rather than wait for another attack, Saitama and Genos decide to visit the House of Evolution themselves, turning the hunters into the hunted.
Episode 3
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Armored Gorilla explains that Dr. Genus devoted his life to forcing human evolution, eventually cloning himself and building the House of Evolution. Saitama and Genos reach the organization's tower and destroy it before the defenders can properly deploy, forcing Genus underground with his most dangerous creation, Carnage Kabuto. The creature demonstrates his strength by demolishing Genos and then senses something terrifying in Saitama, briefly refusing to approach him. When Genos asks how his master became so powerful, Saitama finally reveals his training program: one hundred push-ups, one hundred sit-ups, one hundred squats, a ten-kilometer run every day, no air conditioning, and three years of persistence. Genos and Genus reject the explanation as absurdly ordinary. Enraged by the anticlimax, Kabuto enters a week-long berserk state and attacks. Saitama suddenly remembers that the day is Saturday and that he will miss a supermarket sale. His frustration fuels a single punch that destroys Kabuto. Dr. Genus abandons his research after recognizing that Saitama reached a level his artificial evolution cannot explain. Saitama and Genos sprint away, less concerned with scientific history than discounted groceries.
Episode 4
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Hammerhead leads the Paradisers, unemployed extremists wearing stolen powered suits, in an attack on businessman Zeniru's tower. Their manifesto demands a society where no one has to work, but their destruction mainly creates panic and gives bald people a bad reputation. Mumen Rider confronts them and is quickly defeated. Zeniru sends Speed-o'-Sound Sonic, a highly trained ninja, who eliminates most of the group with ease. Hammerhead survives because his thick skull stops a blade and escapes, only to encounter Saitama. Irritated that witnesses may confuse him with the terrorists, Saitama destroys Hammerhead's suit but spares his life. Sonic then mistakes Saitama for another Paradiser and attacks. His speed and elaborate techniques fail, and an unfortunate collision with Saitama's fist leaves him humiliated. Sonic declares Saitama his permanent rival before retreating. Hammerhead later attempts to seek honest work after the suits' owners nearly kill him for stealing their technology. Saitama, meanwhile, worries that no one officially recognizes his heroic activities. Genos points out that public legitimacy requires registration with the Hero Association. The two decide to apply, giving Saitama a possible route toward recognition—though not necessarily respect.
Episode 5
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Saitama and Genos take the Hero Association entrance examination. Genos earns perfect marks and enters S-Class, while Saitama breaks every physical record but performs poorly on the written test, placing near the bottom of C-Class. At orientation, A-Class hero Snakebite Snek resents Saitama's indifference and later attacks him to establish dominance over a newcomer. Saitama knocks him aside without effort. Genos, now formally recognized as a major hero, asks Saitama for a serious sparring match. He unleashes upgraded weapons and moves at extreme speed, but none of his attacks threatens his teacher. When Saitama finally responds seriously, the pressure of his stopped punch tears apart the landscape behind Genos without touching him. The demonstration confirms the impossible gap between them, yet Saitama immediately changes the subject and suggests lunch. Amai Mask, the highest-ranked A-Class hero, privately warns Genos that power without responsibility can corrupt the Association's image. Genos responds by moving into Saitama's apartment and paying rent, making the apprenticeship unavoidable. Saitama has gained official status and a devoted student, but his low rank means the organization still has no idea how strong its newest member actually is.
Episode 6
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Saitama learns that C-Class heroes must report regular activity or be removed from the Hero Association, so he desperately searches for a crime to stop. Speed-o'-Sound Sonic appears demanding a rematch, but Saitama refuses to participate. Tanktop Tiger accuses Saitama of causing trouble and tries to claim public attention, only for Sonic to attack him as an interruption. Sonic then creates enough chaos to force Saitama's response and is knocked unconscious by a simple strike to the back. Elsewhere, reports describe a powerful monster hiding in the nearly abandoned City Z. A-Class heroes Golden Ball and Spring Mustachio investigate and encounter Kombu Infinity, whose hardened seaweed-like tendrils defeat them. Their failure strengthens rumors that the empty district contains an unknown, exceptionally dangerous being. Kombu Infinity searches for that rumored monster as well, but runs into Saitama carrying groceries. Saitama kills the creature offscreen and brings its tendrils home to improve his soup. Genos later celebrates his own high popularity ranking despite being only seventeenth in S-Class. Saitama, who receives little admiration despite doing most of the work, becomes visibly jealous. The feared presence haunting City Z is, unknowingly, Saitama himself.
Episode 7
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A meteor suddenly changes course toward City Z, creating a disaster too large for normal evacuation. Genos responds alongside veteran martial artist Bang, while Metal Knight arrives mainly to test a new weapons system. Metal Knight's missiles fail, and Genos pours all remaining energy into his own cannon without changing the meteor's path. Saitama reaches the scene at the final moment and shatters the object with one punch. The city is spared a direct impact, but countless fragments cause enormous property damage. No one dies, yet public anger focuses on the destruction rather than the lives saved. Saitama's rank rises sharply, drawing the hostility of Tanktop Tiger and Tanktop Black Hole. They persuade a crowd that he stole credit and caused the damage through reckless action. After defeating the brothers, Saitama openly tells the crowd that he did not become a hero for their approval and will not apologize for saving them. Genos admires his refusal to perform false humility. The crisis confirms Saitama's ability to stop a city-ending threat, but official rankings and popular opinion still distort what happened. Meanwhile, strange creatures gather near the sea, signaling that another disaster is already beginning.
Episode 8
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The Sea Folk emerge near City J and declare war on humanity. A-Class hero Stinger defeats several invaders before their ruler, the Deep Sea King, appears and crushes him. Saitama, Genos, and Mumen Rider head toward the coastal city, though they become separated while searching. Lightning Max and Puri-Puri Prisoner challenge the king and are defeated. Speed-o'-Sound Sonic fights more effectively, but rain restores the monster's full strength and forces the unarmed ninja to retreat. The Deep Sea King then reaches an emergency shelter filled with civilians and injured heroes. Snakebite Snek and several lower-ranked heroes attack despite knowing they are outmatched, but each falls quickly. Genos scouts ahead while Saitama repeatedly misses the battlefield, distracted by incomplete directions and Sonic's sudden appearance. Mumen Rider responds to the Association's call even though he understands the difference in power. Saitama eventually finds a discarded phone and contacts headquarters, his normally casual manner turning serious when he realizes civilians are trapped. The episode ends before the decisive rescue, emphasizing the succession of heroes who stand against the monster not because they expect to win, but because no stronger defender has arrived yet.
Episode 9
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Genos reaches the shelter and initially drives the Deep Sea King back with speed and firepower. The monster adapts, tears off one of Genos's arms, and launches acid toward a child. Genos shields her with his body and is left critically damaged. Mumen Rider arrives next. He has no realistic chance of victory, but his insistence that effort and duty still matter inspires the frightened crowd. The Deep Sea King beats him repeatedly until Saitama finally appears and catches the fallen hero. One ordinary punch destroys the monster and clears the storm. The survivors celebrate, but a cynical civilian argues that the other heroes were useless because Saitama ended the fight so easily. To protect their reputations, Saitama pretends he merely finished an enemy they had already weakened and accepts the crowd's anger as a fraudulent opportunist. His rank rises to B-Class, yet most of his mail is hostile. One sincere letter thanks him. At an oden stand, Mumen Rider reveals that he wrote it and buys Saitama dinner. Their meeting quietly confirms that the person whose respect matters most understood the sacrifice. Elsewhere, a dying fortune teller leaves a warning that Earth itself faces imminent danger.
Episode 10
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The Ancient King appears and is immediately destroyed by Tatsumaki, illustrating the concentration of power among the highest-ranked heroes. Bang invites Saitama and Genos to his dojo and describes Garou, a former disciple who defeated the other students after turning against him. An emergency summons interrupts the conversation. At Hero Association headquarters, the S-Class roster gathers to hear the final prophecy of Madame Shibabawa: Earth will face a catastrophic threat within six months. Before anyone can develop a plan, alien attackers arrive. The Dark Matter Thieves' enormous ship annihilates City A in a single bombardment, killing much of its population. Melzargard, a regenerating alien with multiple heads, attacks survivors on the ground. Iaian loses an arm before Atomic Samurai, Bang, Metal Bat, and Puri-Puri Prisoner engage the creature. Other heroes debate how to reach the vessel, but Saitama simply jumps aboard and begins destroying corridors and soldiers from within. He kills the supposedly elite Groribas almost immediately and continues toward the command center. The invaders believe their attack has overwhelmed Earth's defenses; they do not yet understand that the least celebrated hero in the room has already entered their ship and is approaching their leader, Boros.
Episode 11
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Melzargard repeatedly regenerates against Atomic Samurai, Bang, Metal Bat, and Puri-Puri Prisoner until Metal Bat discovers that each head contains a marble acting as a core. Destroying those marbles permanently removes parts of the creature. A detached head tries to request bombardment support, but the heroes intercept it. Inside the ship, telekinetic commander Geryuganshoop attempts to trap and redirect Saitama with intensified gravity and high-speed projectiles. Saitama dismisses the display as inferior to simply throwing stones and kills him by flicking a pebble through his body. The aliens launch a massive artillery strike at the heroes below. Tatsumaki stops the shells in midair and hurls them back, severely damaging the vessel. Mumen Rider and other heroes concentrate on evacuating civilians rather than seeking glory. Bang is briefly knocked down, but the remaining fighters continue destroying Melzargard's cores. Deep inside the collapsing ship, Saitama finally enters Boros's chamber. Boros explains that a prophecy sent him across the universe to find an opponent capable of giving him a real battle. Saitama's opening punch breaks the armor that had restrained Boros's full power, setting up the confrontation both men have been seeking for very different reasons.
Episode 12
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Freed from his armor, Boros attacks with enough force and speed to survive exchanges that would end any ordinary battle. His regenerative body repairs severe damage, and his released energy tears through his own ship. For a moment, Saitama appears to have found an opponent who can continue after one punch. Boros kicks him all the way to the moon, but Saitama calculates the return jump and crashes back through the vessel. On the ground, the S-Class heroes destroy Melzargard's last core while Tatsumaki and Tanktop Master help force the ship down. Boros uses a final attack intended to erase the planet's surface. Saitama answers with a Serious Punch, dispersing the energy and fatally overwhelming him. Boros dies acknowledging that the fight was never truly close; Saitama had held back throughout. Amai Mask arrives afterward, condemns the heroes for City A's destruction, and executes surviving aliens. Metal Knight salvages the ship to rebuild Association headquarters. Tatsumaki berates Saitama as an unauthorized intruder until Genos defends him. Despite defeating a universe-conquering enemy, Saitama receives no grand recognition. Soon afterward, another monster announces itself, and he returns to the familiar disappointment of ending the threat with one punch.