Sea Monster Arc

Seventh in the series and the fourth arc of the Hero Association Saga, this chapter pits a wave of oceanic invaders against a succession of heroes who each fall short. Its emotional core is Mumen Rider, a powerless C-Class hero who refuses to retreat, and its payoff is both Saitama's effortless victory and the sour public reception that follows.

A towering being surfaces from the ocean, proclaims itself one of the Seafolk, and orders humanity to yield the land, only for Saitama to appear and end it at once. Soon a great wave delivers more Seafolk into the city, where the A-Class hero Stinger rescues a captured citizen and challenges the rest. Meanwhile Saitama and Genos relax at home discussing their rising ranks, until the Association calls about an attack on J-City and Saitama, turning serious, insists they hurry.

Alone and battered, Stinger cuts down several large Seafolk and begins to celebrate, only to be struck down instantly by the Deep Sea King. The city raises the threat to Demon level and urges evacuation. Lightning Max is overwhelmed and nearly crushed before Puri-Puri Prisoner saves him, having been trailed by Speed-o'-Sound Sonic. Puri-Puri Prisoner, thrilled by the king's aura, fights and even shifts into his Angel Style, yet still loses. Sonic then takes his turn, and as rain begins to fall his speed appears to have the upper hand.

The Deep Sea King surges in size, strength, and speed, and Sonic escapes his grasp only by leaving his clothing behind, vowing their next meeting will be the monster's last. Turning toward a shelter, the king punches through its wall, but a run of heroes, All Back-Man, Jet Nice Guy, Bunbunman, and Sneck, step up to hold him off. Mumen Rider, newly arrived, drops his phone on the way to the emergency center, and Saitama picks it up and pries the monster's location from a reluctant Association assistant.

The shelter's defenders are quickly overrun until Genos arrives and drives the king outside, though the monster shrugs off the blow and tears away Genos's arm. When a cheering child draws the king's fury, Genos throws himself in front of a spray of acid, his body dissolving as he shields the girl. The king hurls him into the rain and moves in for the kill, but Mumen Rider ambushes from behind by flinging his own bicycle at him.

Batted aside with ease, Mumen Rider grabs the king again and admits he knows he cannot win, insisting only that someone has to stand and fight. The crowd rallies behind him right up until a single punch knocks him out, and Saitama catches him before he hits the ground. Praising the man's courage and relieved to find Genos still alive despite his melted state, Saitama tells his pupil to hold on. The king, now in his true form, lands a blow on the back of Saitama's head that does nothing at all.

Afterward a care package brings a stack of letters, most praising Genos while heaping scorn on Saitama, which leaves Genos furious on his mentor's behalf even as Saitama waves it off. A single grateful note does reach him. Called in and congratulated for topping C-Class, Saitama opts to rise into B-Class and sits a psychological screening while staff and Sweet Mask debate whether he is a fraud. At a nearby food stall he discovers that the kind letter came from Mumen Rider, and the two share a smile over a bowl of mozuku.

Sonic slips out of prison, and Saitama advances to B-Class. Mumen Rider comes to grasp just how powerful Saitama truly is. Puri-Puri Prisoner, Lightning Max, and Sneck each resolve to grow stronger after their beatings at the hands of the Deep Sea King, while All-Back Man walks away from the Hero Association altogether.