The third season of One Punch Man. Saitama is a hero who only became a hero for fun. After three years of "special training," he’s become so strong that he's practically invincible. In fact, he's too strong—even his mightiest opponents are taken out with a single punch. Alongside Genos, his faithful disciple, Saitama performs his official hero duties as a member of the Hero Association. One day, monsters claiming to be from the Monster Association suddenly appeared, taking a child of Hero Association executive hostage. The S-class heroes gather and plan a raid on the Monster Association hideout to rescue the hostage. Meanwhile, Garou, a "human monster" who was taken by the Monster Association during a battle with the heroes, awakens in the Monster Association hideout.

The inhabitants of a certain wealthy town think nothing of throwing objects away. However, their waste is priceless to Rudo, a resident of the town's slums. Despite the constant warnings from his adoptive father Regto, Rudo spends his days searching for reusable materials that would otherwise be sent to the giant disposal area known as the Pit. Due to its vastness, the Pit doubles as a means of criminal punishment; those dropped in are never to return again. When Regto is murdered by a mysterious assailant, Rudo is falsely accused of the crime and thrown into the Pit. To his surprise, he awakens in a trash-filled area inhabited by enormous monsters formed from the junk. As the toxic air and Trash Beasts push Rudo to the brink of death, he is saved by Enjin, one of the Cleaners who wield weapons known as Vital Instruments to fight the monstrosities. Having gained his own Vital Instrument, Rudo soon joins the Cleaners in the hopes of finding a way to escape the Pit and avenge his father.

Despite being one of Zebrudia's few level 8 treasure hunters and the leader of the capital's top party, Grieving Souls, Krai Andrey does not deserve his prominent reputation. Everything he says somehow turns prophetic, which convinces everyone around him that he possesses unfathomable wisdom regardless of his utter incompetence. Krai knows his success is only a series of coincidences, but the weight of those expectations grows heavier as time goes by. Unfortunately, as the other members of the Grieving Souls continue to shock the world with their achievements, Krai's wish for a peaceful retirement drifts ever further out of reach.

Despite occasional setbacks, Operation Strix has remained on track to extract sensitive information from the enigmatic politician Donovan Desmond. To this end, spy Loid Forger has managed to maintain his pretend family consisting of his telepathic daughter, Anya; assassin wife, Yor; and their clairvoyant dog, Bond. Still the only one aware of everyone's true identities, Anya tries to sustain the family's cohesion while working to befriend Donovan's son Damian, her Eden Academy classmate. However, Anya's efforts instead earn her another Tonitrus Bolt, bringing her closer to getting expelled and jeopardizing the operation's success. Hearing the news, Loid faints from shock. In his unconsciousness, he reminisces about his tragic past and his journey to becoming a spy. Realizing what Operation Strix means to him and the world, Loid finds renewed motivation. Although they each work toward their own covert goals, the Forgers continue to cherish their chaotic yet blissful family life.