The King of the Deep Sea

Shinkai no ō (深海の王)

A Seafolk messenger announces 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 Rider looks Saitama up on the internet and sees that he is powerful, but opinions are divided on him.

At the coast, several large Seafolk monsters emerge from the sea, threatening J-City. The A-Class hero Stinger challenges them with his powerful Bamboo Shoot weapon. Meanwhile, Genos is drying dishes with his built-in equipment as Saitama watches a news report about Stinger's battle against the Seafolk. The two decide to head out to confront the threat. Back at the fight, Stinger takes out the last four Sea-Folk with a Gigantic Drill Stinger Quadruple Thrust attack, but is then bested by the Sea King, the leader of the Seafolk. Saitama, Genos, and Mumen Rider all set off to confront the threat. Genos splits off from Saitama to scout ahead. Various other powerful humans confront Sea King to no avail, including Lightning Max, Puri-Puri Prisoner, and Speed-o'-Sound Sonic. Sonic fights the Sea King to a standstill, until the Sea King is moistened by the rain, thus unleashing his true form. 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 Rider gets a phone call from Hero Association headquarters about the location of the Sea King and decides to go confront the monster, who has just broken into an emergency shelter and has taken down the heroes inside, including Sneck, and is dispatching the heroes there. Saitama returns to where he met Mumen Rider to find a discarded cell phone.