The S Class Heroes

S Kyū no Hīrō (S級のヒーロー)

Child Emperor uses his robot Underdog Man No. 22 to analyze a monster named Eyesight's poisons to find a cure for the paralysis. Child Emperor then proceeds to detonate Underdog Man when it's near Eyesight, but that did not successfully kill her as she hardened her skin. Eyesight then manages to find Child Emperor, but is interrupted and eaten by Pig God (S Class Hero Rank 10). Pig God decides to go after the other monsters in Y-City afterwards. Garou tries to go after Watchdog Man (S Class Hero Rank 12), but a monster beats him to it. Drive Knight ( S Class Hero Rank 9) interrogates a monster he spared after killing the others.

Death Gatling (Class A Hero Rank 8) gathers other heroes to fight Hundred Eyes Octopus, but Flashy Flash (S Class Hero Rank 13) arrives and grievously wounds it with his incredible speed. Right when Flashy Flash is about to unleash his final move, Tatsumaki (S Class Hero Rank 2) arrives and kills Hundred Eyes Octopus with ease, berating Flashy Flash for being too slow before leaving, but not before Flash insults her back. Amai Mask kills monsters in his concert, and Mumen Rider and Tank Top Master defeat monsters in the hospital. Atomic Samurai attends a meeting with the Council of Swordsmasters (Nichirin, Amahare, Zanbai, and Haragiri) to discuss the Garou problem.

Haragiri reveals that he has sided with the Monster Association, and gives the sword masters 3 monster cells which will amplify their strength, speed and skill at the cost of their humanity and turning them into monsters. Haragiri tries to kill Atomic Samurai (as Atomic Samurai is too much of a threat) but Atomic Samurai slays Haragiri before leaving with his disciples (Bushidrill, Iaian and Okamaitachi) to find the Monster Association base. Meanwhile in the Super Fight Tournament, Saitama defeats Sourface and Choze before facing off against Suiryu in the finals, with Suiryu promising to show Saitama what martial arts are all about.

During the fight Suiryu begins to express his disdain about the hero lifestyle, and tries to persuade Saitama not to become a hero since it is pointless and boring to do heroic justice and save the day. Instead, Suiryu remarks that people with great strength such as Saitama should seek thrills, just like him before he kicks Saitama in the head. Angered and annoyed, Saitama lets the kick connect, knocking the wig off his head. Finally exposed, Saitama throws a short range punch while exclaiming that if Suiryu's goal is to seek fun, he shouldn't try to make himself stronger than he is currently.

However, Saitama restrains his punch before it hits Suiryu, which creates a powerful shock wave that blows off Suiryu's clothes, exposing his muscular body and leaving him in a state of deep shock. Despite Saitama being disqualified, Suiryu still attacks Saitama in anger, even using his final attack (Void Phoenix Ascension Fist). Saitama is unfazed by the assault. When Saitama says that martial arts are techniques that make you look cool, and starts spinning around, Suiryu tries to attack Saitama yet again but is knocked back by Saitama into a wall while taking Saitama's white belt, resulting in Saitama's pants falling down.

Despite Suiryu embarrassing Saitama with that action, he is still in deep shock that he lost. In the post credit scene, Genos wakes up from the wreckage, and a monster is heading toward the Martial Arts Tournament.