The Deep Sea King

A herald of the Seafolk proclaims that his kind are rising to the surface to exterminate humanity. Passing by on his way back from errands, Saitama silences the messenger with a single strike. Mumen Rider arrives too late to see anything but the wreckage and the buzzing crowd, so he looks Saitama up online, finding a powerful but divisive figure.

Along the coast, hulking Seafolk climb out of the water and menace J-City. The A-Class hero Stinger takes them on with his Bamboo Shoot weapon and skewers the last several with a rapid multi-thrust attack, only to be crushed by the Sea King, the group's leader. Watching the televised battle while Genos dries the dishes, Saitama decides they should head out. On the way, one fighter after another challenges the Sea King and fails, among them Lightning Max, Puri-Puri Prisoner, and Speed-o'-Sound Sonic. Sonic manages a deadlock until the falling rain soaks the monster and triggers his true, far stronger form, forcing the ninja to flee.

Saitama and Mumen Rider cross paths en route, and Saitama peels off after spotting Sonic sprinting through the downpour with no clothes on. Mumen Rider takes a call from headquarters pinpointing the Sea King, who has already smashed into an emergency shelter and is cutting down the heroes stationed there, Sneck among them. When Saitama circles back to where the two had met, he finds only an abandoned cell phone.

The Seafolk invade the surface and Saitama destroys their messenger. Stinger is defeated by the Sea King, and Lightning Max, Puri-Puri Prisoner, and Sonic all fall in turn. Rain unleashes the Sea King's true form. The monster breaks into an evacuation shelter and overwhelms the heroes inside, setting up the crisis Mumen Rider races toward.

The episode covers chapters 23 through 26 and opens the Sea Monster Arc. Stinger and Lightning Max share the closing monologue over the credits. The adaptation invents several beats, including Mumen Rider riding to the first Seafolk, his online research on Saitama, and the meeting between Saitama and Mumen Rider on the road, while trimming the shelter fight so only the heroes' defeat is shown.