One-Punch Man season 1

A monster named Vaccine Man wreaks havoc in a city and is about to kill a young girl when a bald hero named Saitama intervenes and kills him with a single punch. Saitama recalls the events three years earlier that pushed him to become a hero; after saving a boy with a cleft-chin from Crablante, Saitama resolved to become a hero but became too powerful for his own satisfaction. Following taking out the Brain and Brawn Brothers, one of whom is a mutated giant, Saitama has a dream about a subterranean race who invades the Earth's surface and actually pose a challenge to him, rekindling a passion for battle he had been missing for years. However, when Saitama wakes up, he finds that the real Subterranean race is extremely weak. He kills their leader, and the rest immediately retreat.

A cyborg named Genos tracks down a mutant called Mosquito Girl, whose mosquito swarm drains the blood out of everyone they come across within the city. Genos has a difficult time with Mosquito Girl who almost forces him to activate his self-destruct as a last resort to stop her. Saitama then suddenly stumbles into the fight, easily killing Mosquito Girl. Genos tells Saitama how a mad cyborg destroyed his hometown and killed his family. Despite Genos's extremely boring explanation of his past, Saitama agrees to make Genos his apprentice. Soon after, Mosquito Girl's fellow mutants from the House of Evolution try to capture Saitama, with only Armored Gorilla surviving as he agrees to answer the heroes' questions.

Armored Gorilla tells the story of Doctor Genus, the House of Evolution's founding leader who wants to study Saitama's power to replicate it. Saitama and Genos reach the House of Evolution's base, forcing Genus to release his strongest creation: the unstable Carnage Kabuto. After Kabuto effortlessly defeats Genos, Saitama is eager to fight him, but Kabuto cowers upon sensing Saitama's limitless power. Saitama finally explains the secret to his strength-a simple workout regimen, much to Genos and Genus's astonishment. This infuriates Kabuto as he transforms into his berserk form, only to be killed when he unwittingly makes Saitama realize that he missed out on a big supermarket sale. Saitama and Genos rush to the sale while Dr. Genus abandons his research.

The hard-headed criminal Hammerhead forms the Paradisers terrorist cell to demand free goods for the unemployed, targeting local business magnate Zeniru using stolen enhancement suits. While they easily defeat bicycle hero Mumen Rider, Hammerhead's men are taken out by Zeniru's hired ninja Speed-o'-Sound Sonic. Hammerhead flees after surviving a kunai stab to the back of his head, only to run into Saitama, who clarifies his lack of association with the Paradisers despite their shared baldness. Saitama easily defeats Hammerhead by destroying his suit, but lets him live and run away. Then Sonic, thinking Saitama was one of the Paradisers, attacks him, but is defeated after the ninja accidentally plants his crotch on Saitama's fist. The ninja quivers in agony and retreats, but not before declaring Saitama his eternal rival. That night, as Hammerhead turns over a new leaf after the owners of the stolen suits nearly killed him, Saitama sulks over his lack of recognition as a hero before Genos suggests they join the Hero Association.

Saitama and Genos pass an exam for the Hero Association, with Genos placing in S-Class while Saitama ends up in C-Class by breaking every record in the physical portion of the exam but doing very poorly in the non-physical sections. They then attend an orientation seminar by Snakebite Snek, who is annoyed that Saitama does not pay attention to him. Snek tries to newbie crush Saitama, but is humbled when Saitama effortlessly defeats him. Having received new upgrades, Genos asks Saitama to spar with him and to do so seriously. Saitama humors him, but when he gets serious he completely outclasses Genos before he stops his finishing blow and calmly asks him to go get some food with him. A-Class Hero Amai Mask meets with Genos to deliver a welcome gift and a warning to take his responsibilities seriously. Genos moves in with Saitama, much to the latter's chagrin.

Saitama tries to come up with ideas for mentoring Genos, then rushes off to find a villain once learning that C-class heroes get dropped off the registry after a week of inactivity. Sonic demands to settle their alleged rivalry, but Saitama refuses. Tank-Top Tiger reproaches Saitama for his antics and tries to steal credit when Sonic attacks him for interfering between him and Saitama’s rivalry. Sonic then starts causing mayhem to force Saitama to fight him; Saitama indeed notices, and swiftly knocks him out with a tap on the back. Later that day, rumors of a monster skulking around the desolate Z-City reach the Hero Association. They send A-Class heroes Golden Ball and Spring Mustachio to investigate before they are taken out by the monster Kombu Infinity. While assumed to be the rumored monster, Kombu Infinity was actually attracted to Z-City by the rumor as well, but Saitama kills the monster and uses its remains for kombu soup. Sometime later, Genos informs Saitama that he is ranked 6th in popularity despite being in 17th place within the S-class, greatly annoying an envious Saitama.

Scientists and astronomers are busy studying the flight pattern of a meteorite that suddenly changed its trajectory-towards City Z, with all S-Class heroes deployed to intercept the meteor. Only Genos arrives on the scene, followed by an aged S-Class hero Bang. He tells Genos the meteor evacuation protocols have begun and that he should clear out. Bang himself cannot leave the city due to personal reasons, but before he can formally introduce himself, Genos leaves, finding it too late to evacuate. Another S-Class hero, Metal Knight shows up to test a weapon against the meteor, uninterested in working with Genos. When Metal Knight fails, Genos uses his own prototype weapon on the meteor, which also fails despite a much better showing. Saitama shows up and destroys the meteor, which shatters into several meteorites and still damages the city, though with zero casualties. The next day, Saitama learns his rank has jumped from 342nd to 5th, and goes out to look for more crises to deal with. Tanktop Tiger appears and claims Saitama has stolen credit; he calls his brother Tanktop Black Hole, who rallies up a mob against Saitama, blaming him for the city's destruction. The brothers attack Saitama, and he makes short work of them. Saitama then moves on to the crowd, who were chanting for his resignation, and shouts everyone down, clarifying that he does not need to be appreciated for his work, impressing a spectating Genos. In the post-credits, unseen citizens discuss Saitama's victory over the meteor, while a myriad of ominous creatures appear to be gathering under the sea.

The Seafolk Tribe openly declare war on humanity. Saitama easily kills an octopus-like monster while on his way home. Mumen Rider is late to the scene and sees only the destroyed body of the creature, and hears the crowd talking about Saitama. Mumen looks Saitama up on the internet and finds that he is powerful, but opinions are divided on him. At the coast, several large Sea-Folk monsters emerge from the sea, threatening City J. The Class A hero Stinger (rank 11) challenges them with his powerful Bamboo Shoot weapon. Meanwhile, Saitama watches a news report about Stinger's battle against the Sea-Folk. Stinger takes out the last four Seafolk, but is then bested by the Seafolks' leader: Deep Sea King. Saitama, Genos, and Mumen Rider all set off to confront the threat. Genos splits off from Saitama to scout ahead. Various other heroes confront Deep Sea King to no avail, including Lightning Max (class A), Puri-Puri Prisoner (class S), and Sonic (not a hero). Sonic fights Sea King to a standstill, but when it starts raining, Sea King unleashes his true form and Sonic is forced to retreat (without his clothes). Meanwhile, Saitama and Mumen Rider meet each other on the way to the fight, but Saitama trails off after seeing Sonic running naked. Mumen gets a call from the Hero Association headquarters and goes to confront Sea King, who has broken into an emergency shelter and is dispatching the heroes there ranging from All Back-Man, Bunbunman, Jet Nice Guy, and Sneck. Saitama returns to find a discarded cell phone. The episode ends with Saitama turning serious as he tells the Hero Association he’s on his way.

Genos initiates the battle and seemingly overpowers the Deep Sea King until the monster rips off one of his arms. He tells the civilians to flee before being badly damaged shielding a child from a glob of acid the Deep Sea King spewed before Mumen Rider appears. Despite his determination and the civilians cheering him on, Mumen Rider is brutally beaten until he is caught by Saitama, who arrives in that exact moment. After exchanging few words with the Deep Sea King, Saitama defeats him with a single punch, stopping the rain in the process and earning the praise of the civilians. However, when one of the civilians starts badmouthing the other heroes, Saitama downplays his victory to restore the peoples' faith in the other heroes at the cost of his own public image. Some time later he and Genos get fan mail, with Saitama getting only one hate mail but also one thank you letter as well as a notification from the Hero Association that his conduct promoted him to 101st Rank, B-class. While on his way home, Saitama encounters Mumen Rider in a roadside oden shop. Mumen Rider reveals he sent the thank-you letter and pays for Saitama's dinner. In the post-credits, a fortune teller named Madame Shibabawa warns an official from the Hero Association that "the Earth is doomed".

The Ancient King of the Terror Lizard Clan appears but is easily destroyed by Tatsumaki. At Bang's dojo, Bang tells Saitama and Genos about how his former best student, Garou, had gone wild and defeated all of his other top students. Suddenly, a Hero Association member comes and informs the heroes that an emergency has been declared and all S Class heroes have been called to a meeting. At the association, Saitama meets Atomic Samurai, an arrogant S Class hero, and the other S Class heroes, Puri-Puri Prisoner, Metal Bat, Tanktop Master, Flashy Flash, Watchdog Man, Superalloy Darkshine, Pig God, Drive Knight, Zombieman, King, Child Emperor, except for Metal Knight and Blast, who are absent. Sitch, a staff member of the association, starts the meeting by telling the heroes that the emergency promises extreme danger: the great seer, Madame Shibabawa, had died while becoming agitated and choking on a cough pill while predicting the future. Sitch mentions that Shibabawa's predictions have always been 100% accurate; she left a note saying "The Earth is in trouble", prompting a reaction of shock and disbelief among the heroes. He warns the heroes to prepare for a fight in the next 6 months. Just after he says this, the HQ is attacked by the Skyfolk led by Sky King, who are quickly slain by Melzalgald, a member of an alien mercenary group called the Dark Matter Thieves. The Dark Matter Thieves' spaceship wipes out City A, causing large casualties as their invasion begins. Just as Melzargard is about to kill some civilians, Iaian, a student of Atomic Samurai, battles him. Saitama encounters the alien ship and advances towards it. Meanwhile, Melzargard proves too strong for Iaian, who loses his left arm before Atomic Samurai saves him. Metal Bat, Bang and Puri Puri Prisoner arrive to back up Atomic Samurai. Elsewhere, the other S Class heroes discuss how to bring down the ship. Genos notes that Saitama has already entered the ship and has slain a large number of the aliens. Saitama encounters and instantly slays Goribas, a seemingly powerful alien. As Saitama continues to search the ship for its leader, Boros is seen on his throne.

Geryuganshoop complains about Groribas' defeat as he and Melzargard are the only elite fighters left alive. He also informs Boros that the ship is badly damaged by the human intruder but Lord Boros tells him not to fear. Meanwhile, Puri Puri Prisoner's efforts are made futile as Melzargard simply regenerates, and sends one of his heads to go and contact the ship. As the head tries to fly away, Metal Bat smashes it down. Atomic Samurai, Puri Puri Prisoner and Bang join forces and attack Melzargard together. Melzargard and Geryuganshoop speak telepathically, and the former requests for a cleansing bombardment. While Melzargard is distracted, Metal Bat finds a marble within one of Melzargard’s heads and crushes it, which destroys the head. Geryuganshoop tries to direct Saitama out of the ship, but Saitama tricks him and heads to the control room. He finds Geryuganshoop, who displays incredible telepathic powers; Saitama nonchalantly throws a pebble at Geryuganshoop and kills him. After busting more heads with Metal Bat's method, a shell bombardment begins. Tatsumaki stops the missiles in mid air, and fires them back at the ship at full force. Mumen Rider helps a victim, and Stinger and Lightning Max appear to help too. Bang finds another marble and crushes it. Angry, Melzargard slaps the concrete along with Bang, who takes a direct hit. He sticks onto a building, unconscious. With one more head left, Melzargard threatens Atomic Samurai, who angrily retaliates with his ultimate technique. Saitama finds Boros, who recognizes him as Earth's strongest warrior and explains he came to Earth because of a prophecy to find his ideal opponent. Saitama punches Boros, but his attack merely destroys the alien's armor, which was designed to suppress his full power.

Boros starts fighting Saitama, and their battle greatly damages the spaceship. As Boros escalates his attacks, he kicks Saitama to the moon. Meanwhile, Mumen Rider, Stinger, and Lightning Max rescue the last City A survivors. Tanktop Master throws a piece of rubble at the ship, which Tatsumaki replicates with dozens of pieces. Bang recovers and destroys the final marble, killing Melzargard. Saitama returns from the moon, crashing the ship aground. Reaching his limit, Boros uses an energy attack capable of destroying a planet; Saitama performs a "Serious Punch" which overwhelms Boros. As Saitama emerges victorious, Boros admits he was not even close to Saitama's strength, and dies. After the fight, Drive Knight warns Genos that Metal Knight is his enemy. Amai Mask appears and blames the S-class heroes for the destruction of City A; Metal Knight arrives, wishing to salvage parts of the ship. Meanwhile, Superalloy Darkshine discovers alien survivors whom Amai Mask promptly executes. Saitama emerges from the ship, resulting in Tornado verbally abusing him, as she believes Saitama had stolen her fight. Genos tells her to shut up while calling her a brat, which causes her to smash him into a nearby rock, and threatens to kill Saitama, but Bang stops her and scolds her for her actions. The parts Metal Knight salvages from the ship are used to rebuild the Hero Association HQ on the remains of City A. Both A and S Class heroes are invited to reside there. In the post-credits, Saitama destroys Pluton: King of the Underworld with one punch, much to his irritation.