The episode opens with an octopus-like monster announcing that the Seafolk are coming to the surface to kill all the humans. Saitama is on his way home from shopping and stops by to punch the messenger. 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 sees 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, Genos is drying dishes with his rocket hands as Saitama watches a news report about Stinger's battle against the Sea-Folk. They decide to head out to confront the threat. Back at the fight, Stinger takes out the last four Seafolk with a Gigantic Drill Stinger Quadruple Thrust attack, but is then bested by a new monster: 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 technically 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. Meanwhile, Saitama and Mumen Rider meet each other on the way to the fight. Saitama goes off to find Sonic who he sees running naked through the rain. Mumen gets a phone call from 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 where he met Mumen Rider to find a discarded cell phone. The episode ends with Saitama telling the Hero Association he is on his way.