Plot
On a supercontinent version of Earth, ravaged by monstrous threats, the Hero Association mobilizes superheroes to defend humanity. Among them stands Saitama, an unaffiliated hero from City Z whose rigorous training regimen granted him invincible strength—capable of ending any battle with a single punch. This absolute power has left him disillusioned, longing for an opponent who can reignite his passion for combat. His solitary existence changes when he encounters Genos, a cybernetic warrior seeking vengeance against the machine that annihilated his hometown. After witnessing Saitama effortlessly defeat the monstrous Mosquito Girl—an adversary who had nearly killed him—Genos becomes his devoted disciple.
The Hero Association's ranking system soon exposes its superficial nature. Despite his godlike abilities, Saitama's poor written exam results relegate him to C-Class, while Genos immediately attains the elite S-Class designation. When Saitama single-handedly stops an asteroid impact and later defeats the amphibious terror Deep Sea King, public opinion turns against other superheroes for their comparative weakness. In characteristic selflessness, Saitama deliberately draws criticism onto himself to protect his peers' reputations.
This cycle of duty and public ingratitude continues until the alien warlord Boros arrives, fulfilling the dying prophecy of seer Shibabawa. While the Association's forces battle Boros's army, Saitama confronts the conqueror in a one-sided duel that ends with his signature finishing move. The victory only deepens his existential crisis—even the "Dominator of the Universe" fails to challenge him.
Simultaneously, the emergence of Garo, the self-styled "Hero Hunter", coincides with escalating monster attacks. A disgraced martial artist scarred by childhood bullying, Garo develops pathological hatred for hero culture. His violent campaign intersects with the Monster Association's schemes—a subterranean organization transforming humans into monsters. When they kidnap a Hero Association executive's son, Garo forms an unlikely bond with Tareo, a hero-worshipping child, before both are captured.
The resulting assault on the Monster Association's headquarters becomes history's most destructive battle, reducing much of City Z to ruins. During the conflict, Saitama encounters Blast, the mysterious top-ranked superhero, who warns of an apocalyptic entity called "God". The battle climaxes when Garo, mutated into monstrous form, receives cosmic power from this entity and slaughters numerous superheroes—including Genos, whose core he destroys to provoke Saitama.
Their subsequent cataclysmic battle spans the solar system before Saitama prevails. In his final moments, Garo teaches Saitama time travel, enabling him to prevent this dark future. Returning to the past, Saitama subdues Garo preemptively, with only Genos retaining memories of the original timeline.
In the aftermath, Saitama's promotion to A-Class coincides with the Hero Association's crumbling credibility. Heroes dissatisfied with the association join the Neo Heroes, a rival group led by the enigmatic Blue—claiming to be Blast's son.
The new faction claims to optimize hero work, forgoing the hero ranking system and unfair treatment of differently ranked heroes. Neo Heroes are also given battle suits, armored vehicles, and proper equipment all while being led by Neo Leaders consisting of former S-Class heroes and other powerful figures.
Unbeknown to most, this faction harbors sinister ambitions of mass cyberization. Amidst all this, Drive Knight reveals to Genos that Metal Knight is responsible for the Mad Cyborg. Subsequently, Dr. Kuseno is assassinated by Machine Gods sent by the Organization, who launch an invasion consisting of millions of robots into every city, igniting a mechanized conflict potentially tied to Genos's past.
