The Sea Monster Arc is the seventh arc of the series, and the fourth arc of the Hero Association Saga.
A large Mysterious Being rises from the sea and begins to terrorize the nearby city. It declares itself to be one of the Seafolk, and that the humans are to surrender the surface to them at once. His rampage is short lived as Saitama arrives on the scene and dispatches him quickly. Later on, a large wave crashes to shore, bringing with it more Seafolk who then head into the city and start to attack. A citizen is grabbed by one of the Seafolk, but is quickly saved by the A-Class hero Stinger, who then challenges the rest of the Seafolk to battle.
The scene changes to Saitama and Genos relaxing in their apartment. Genos is doing dishes and commenting on their recent rises in rank, stating that Saitama could soon be moved up to B-Class, and mentions that the current Rank 1 C-Class hero has been in his position for more than half a year already. At that point, Genos receives a call from the Hero Association stating that the Seafolk had attacked J-City. He mentions this to Saitama who then turns on the news to check on the situation. The news shows a battered Stinger still facing off against the Seafolk. Saitama, now serious, states they need to get there as fast as possible.
An exhausted and battered Stinger stands before several large Seafolk. They tell him to give up, that all his supporters had left and that he was going to die there. Stinger blows off these jabs and states that now that he is by himself he can go all out. One of the Seafolk sends an attack at Stinger who meets it with an attack of his own that blasts a hole through the Seafolk. Leaping through the whole he uses another attack and dispatches the last three Seafolk. He lands and starts to celebrate his victory only to be immediately taken out by the Deep Sea King.
The chapter begins with a city alarm announcing that the disaster level of the Seafolk invasion has been raised to Demon. In addition, the citizens are advised to evacuate the city. The Deep Sea King continues to walk through the city. Lightning Max is seen observing the Sea King from afar. The Deep Sea King then suddenly appears behind Lightning Max and their fight initiates. During the fight is overwhelmingly one-sided and Lightning Max ends up losing. Puri-Puri Prisoner comes in time to save Lightning Max from being crushed by a building. It is revealed that Speed-o'-Sound Sonic has followed Puri-Puri Prisoner and Sonic thanks him for allowing him to escape. Puri-Puri Prisoner becomes excited to fight the Deep Sea King after sensing the King's strong aura. The two then begin fighting while Sonic observes. Puri-Puri Prisoner ends up taking a lot of damage and resorts to using his Angel ☆ Style transformation. However, it is proven to be useless as the Sea King was still able to beat Puri-Puri Prisoner. After Sonic and the Deep Sea King have a small chat, they begin fighting. During their fight, it starts to rain. Sonic's speed seems to be too much for the Deep Sea King.
Sonic's fight with the Deep Sea King continues, but the Sea King gets a noticeable speed, strength, and size boost. Sweet Mask is briefly seen on television, he tells the interviewer a little about his ideology about what a hero is in his mind. The Deep Sea King is later seen chasing Sonic and is able to grab him. However Sonic was able to slip through, leaving his clothes behind. Sonic then tells the Deep Sea King that the next time they meet, it will be the Sea King's last. Sonic bumps into Genos while on his way to get his weapons. Before Sonic leaves, he tries to belittle Genos' hero career. Afterwards, the Deep Sea King decides to go to one of the shelters. He was able to effortlessly make a hole in one of the walls of the shelter. Before he could attack the people C-Class hero, All Back-Man, interrupted him. The B-Class hero, Jet Nice Guy, the C-Class hero Bunbunman, and A-Class hero, Sneck, followed All Back-Man's lead soon afterward. The chapter ends with the crowd cheering for the heroes as the heroes prepare for battle.
Mumen Rider has just arrived at J-City and is talking on the phone with the Hero Association. He is told to rush to the emergency center. As Mumen Rider rides off to the shelter, he drops his phone. Saitama ends up picking the phone and talks to one of the Hero Association assistants. The assistant is initially reluctant to tell Saitama where the monster is, but after looking over Saitama's record, he changes his mind. The heroes at the emergency shelter are quickly defeated by the Deep Sea King. However, Genos soon arrives and starts fighting with the Deep Sea King.
Genos launches the Deep Sea King out of the emergency shelter, but also creates another large hole. Unfortunately, the Deep Sea King immediately recovers from Genos' punch and retaliates by attacking Genos when he was distracted by the crowd. The Deep Sea King is able to rip Genos' arm off and knock him towards a wall. Genos tells everyone to get out of shelter while he holds off the Deep Sea King. During their battle, one of the children cheers on Genos, this annoys the Sea King and causes him to launch a spit of acid at the child. Genos is just barely able to get in front of the spit in time preventing it from harming the child.
As Genos' body melts due to Sea King's acidic saliva, Sea King grabs Genos and throws him against the wall of the shelter and proceeds to punch him through the wall out into the rain. Sea King expresses his surprise at the fact that Genos sacrificed himself to save a little girl. Just as he is about to deal the finishing blow, Mumen Rider attacks him from behind with his Justice Crash, by literally throwing his bicycle at the Sea King.
As Genos attempts to stop Mumen Rider, he attacks the Sea King but is easily parried and smashed to the ground a few times before getting thrown away. Sea King then returns to finish off Genos, but Mumen Rider intervenes again by grabbing the Sea King from behind. Mumen Rider admits that he knows better than anyone that he has no chance of beating the Sea King, however, he states that it does not matter if he stands a chance or not, he has to fight him since there is no one else who will. This causes the people to start cheering for him and he gets pumped up, but is knocked out by the Sea King with a punch to his face. Saitama suddenly appears at the scene and grabs Mumen Rider before he falls to the ground. Saitama praises Mumen Rider's fight and sets him to the ground, he spots Genos lying there and is surprised that he is still alive, despite his melted state. He then tells Genos to wait a little while longer while he takes care of the 'sea freak'. Sea King, who has transformed into his true form, tells Saitama that he can hear him and punches him in the back of his head. Saitama, however, appears to be unaffected.
A care package is dropped by Saitama's home. Inside the the care package is a bunch of letters from various people. Almost all of letters for Genos are positive, while the majority for Saitama are negative. Genos starts to feel very angry for the lack appreciation that Saitama is given for the countless lives that he has saved. Although Saitama simply shrugs the hatred off, Genos feels deep regret for showing Saitama the letters. However, Saitama eventually receives a thank you letter. Saitama is called in by the Hero Association to congratulate him for becoming rank one in C-Class. Saitama brought into an interview to ask him whether he wants to remain in C-Class or move up to B-Class.
Saitama decides to move up to B-Class and is given a psychoanalysis test. Overhead of the interview, there are several Hero Association staff observing and discussing about Saitama. They discuss whether or not he is a sham. After a short discussion, Sweet Mask is seen as an adviser on whether or not Saitama should move up. Sweet Mask replies that they do not need him for such as trivial task as promoting someone to B-Class. Sweet Mask additionally states that he only wants a say in promoting people to Class A and higher as they tend to have a strong influence on the association's image. After the interview, Saitama goes to a nearby a food stand to eat. There he finds Mumen Rider and it is revealed that it was he who wrote the thank you letter to Saitama. The two share a smile and order mozuku.
- Sonic escapes prison.
- Saitama is promoted to B-Class.
- Mumen Rider becomes aware of Saitama's power.
- Puri Puri Prisoner, Lightning Max and Sneck seek to become stronger after their defeats at the hands of the Deep Sea King.
- All-Back Man quits the Hero Association.
- The first look into the evacuation center shows several copy/pasted individuals to give the sense of how crowded it is. Upon closer inspection, there are actually fewer people in there.