Unyielding Justice

While Saitama waits on the association to clear him for backup, Genos attacks the Deep Sea King and nearly puts it down, but lowers his guard and loses an arm. He orders civilians to flee and buys time, holding his own until the monster hurls a spray of acid at a small girl. Genos shields her, and the acid ruins his body and leaves him near death. Just before the finishing blow lands, Mumen Rider arrives and throws himself at the Sea King. He is battered aside repeatedly yet keeps standing, and the crowd, moved by his refusal to quit, begins cheering him on. When he finally collapses, Saitama catches him, having been cleared for backup at the exact moment he was needed, and ends the fight with one punch to the townspeople's applause. In another district, Sonic reappears and assumes the monster fled.

Back home, a delivery of fan mail brings praise for Genos and an insult for Saitama, who is branded a cheat. A flashback reveals why: right after the kill, a bystander began deriding the fallen heroes as frauds, reasoning that a monster beaten by a single C-Class punch could never have been dangerous. To shield their names, Saitama laughed off his own feat, claiming he arrived late against an already weakened foe and got lucky, and urged the crowd to honor the heroes who came before him. They accept him as a fraud and thank the others. Genos chooses not to interfere but vows to step in should the choice ever corner his master.

In the present, Saitama finds another note reading simply "Thank You!" and a third informing him he is now C-Class rank one and invited to discuss promotion to B-Class, which he accepts, landing at rank one hundred and one. Sweet Mask shrugs off the news until he learns Saitama actually slew the monster. Lightning Max and Stinger recover in a hospital while Puri-Puri Prisoner, dubbed an S-Rank failure by the papers, tries to visit them. Walking home that night, Saitama runs into Mumen Rider at an oden stand, who admits he sent the thank-you note and covers the meal. After the credits, the seer Shibabawa warns an association official that the Earth is doomed.

Genos is gravely wounded shielding a child, and Mumen Rider's stubborn stand against the Deep Sea King rallies the crowd before Saitama arrives to kill the monster in one punch. To protect the defeated heroes, Saitama publicly dismisses his own win, earning a reputation as a fraud. He is promoted to B-Class rank one hundred and one, and Shibabawa foretells the Earth's doom.

The episode adapts chapters 26 through 29 and closes the Sea Monster Arc, with the Deep Sea King delivering the end-credits monologue. The cheering child is the same one Mumen Rider helped retrieve a balloon for in an earlier episode. Anime-original touches include Genos punching the monster out of the shelter, the fan mail arriving via drone, the hospital scene with the recovering heroes, and the near-miss where Saitama and Mumen Rider almost get run over.