The Alien Conquerors Arc is the eighth arc of the series, and the fifth arc of the Hero Association Saga.
Bang is shown demonstrating his technique, Water Stream Rock Smashing Fist, to Saitama and Genos. He asks if they would like to give the technique a try. Unfortunately, they decline Bang's offer. Charanko is angered by their decline and challenges to fight them. Genos quickly defeats Charanko and states that he underwhelmed by the lack of skilled fighters in Bang's dojo. Bang explains that his dojo used to be full of skilled disciples, but they were all disabled by Garou, Bang's best disciple in the past. Bang further tells them that he expelled Garou after giving him a beating. Saitama acknowledges Bang's strength; a statement which Charanko would then scorn Saitama for because of Saitama's lack of knowledge of Bang's popularity.
After Charanko's constant talk, a Hero Association employee arrives at Bang's front door informing Bang and the others that an emergency summons has been issued to S-Class heroes. Bang, Saitama, and Genos leave for the meeting, while Charanko is left to watch over the dojo. At the headquarters, they are greeted by Atomic Samurai. Atomic Samurai declines Saitama's offer for a handshake, stating that he will greet him when he becomes a S-Class hero. Tatsumaki suddenly starts shouting at Saitama and belittling him only to have Genos inform Saitama that she is rank two in the S-Class and is an Esper. At the meeting, each S-Class member gives off their thoughts on the matter. Saitama asks for tea.
Sitch introduces himself and explains the situation. Before he explains the situation, he warns them that once they start listening to what he has to say, they will be confined to the room until the meeting is over. Metal Bat interrupts Sitch, stating that he had to skip his sister's piano concert to attend this meeting, and threatens to destroy the headquarter building if Sitch did not have anything important to say. Sitch announces that the great prophet, Shibabawa, is dead. Most of the heroes are shocked to hear this news. Zombieman asks whether she was murdered, which Sitch replies that she just died from choking on cough drops. Superalloy Darkshine asserts that the Hero Association brought them to tell them that they must now prevent disasters without Shibabawa's help. Sitch states that Darkshine is wrong and that she has always only predicted a fraction of all disasters. Saitama asks Puri-Puri Prisoner who Shibabawa is.
Puri-Puri Prisoner tells Saitama that she was a great prophet that could predict the exact timing of disasters, such as natural disasters and monster attacks. Finally, Sitch states that the real problem was that her final prophecy was a note stating the Earth is in trouble. Child Emperor then tries to make an excuse to leave. However, he is only mocked by Sitch, stating that Child Emperor is nothing more than a child if he does not comprehend the danger of this matter. Additionally, Sitch states that the disaster is above Dragon and will happen within half a year. Watchdog Man comments that they cannot do much to prevent the disaster. Saitama remarks that the prophecy might come true sooner than expected. Suddenly, the headquarters comes under fire from Skyfolk. Outside the Sky King prepares his attack with his henchmen. However, the Skyfolk are then attacked by an unseen being.
A large craft is seen flying over A-City, where it stops and proceeds to destroy the entire city. Sky King is then seen, dead, his body falling apart as a large creature with several heads and wings comments to itself on killing the Sky King, and the destruction caused by the ship. Iaian, a hero, then crawls out of the rubble and witnesses the state of destruction of A-City, the only thing left standing is the Hero Association's headquarters. He then notices the giant ship above him, but only looks for a second before the large monster appears behind him and attempts to crush his head. The monster only crushes his helmet, its occupant having moved safely away. Iaian comments on how there would be no way he would not notice the monsters killing intent and then proceeds to attack it.
Sitch is screaming, and after seeing that A-City was destroyed in an instant, he is certain the "earth is in danger" prophecy had come true. Bang asks Sitch why the Hero Association building is still standing, to which he is answered with the fact that Metal Knight had built it, and that it is much stronger than an average shelter, Metal Bat then notes that because it is built like a bomb shelter, the place has no windows. The heroes are preparing to leave to deal the threat outside, and just as Genos is about to ask Saitama to join them, he notices that his master is not there and that a hole in the "impenetrable fortress" ceiling has been made, apparently all the way to its roof.
Saitama is then seen on the roof, preparing to deal with the threat in front of him, a giant spaceship. Saitama leaps towards the spaceship and is shot with a colossal-size bullet, which he is able to dodge, and is also relieved, due to the fact that if the weapon was a laser beam, it would've burned off his clothes. Saitama is then once more shot with gigantic bullets, but repels them and sends them back to their own ship.
Meanwhile, Iaian is still fighting the alien Melzargard, who appears invulnerable to his attacks. The alien then retaliates and demolishes the area behind Iaian, as well as his arm. The alien then proceeds to split himself, into 5 different forms.
The creature then proceeds to split itself into more individuals and proceed to attack Iaian, however, they are stopped by Atomic Samurai who shows concern for Iaian's injuries. The creature regenerates, and Atomic Samurai engages it in a fight. Once it seems that Atomic Samurai has gained the upper hand the individuals combine to form one single entity who seems to be excited at the prospect that someone can match them in power. Bang, Metal Bat and Puri-Puri Prisoner arrive to aid Atomic Samurai take on Melzargard.
S-Class heroes are then seen debating on how to deal with the flying ship, Superalloy Darkshine suggests King takes on the ship to which King replies that he is unable to do so, hearing this angers Tatsumaki who then decides to stop the ship by herself.
Saitama is then seen having already boarded the ship in an altercation with Groribas who is easily defeated in one punch before even given the chance to attack. He then decides to destroy the ship from the inside much to the dismay of Geryuganshoop who is shocked by the power Saitama possesses. Boros then appears and is then shocked after learning of Groribas' defeat.
As the S-Class heroes battle the monster, Melzargard was struck by Bang and decide to contact the ship to prepare the cannons. He creates a small flying head to contact the ship, but the head was struck by Metal Bat. Geryuganshoop then contacts Melzargard to inform him someone intruded into the ship.
Metal Bat, Bang, Puri-Puri Prisoner, and Atomic Samurai continue their battle with Melzargard. Iaian suggests to retreat, but the heroes refuse to listen. Melzargard is then contacted by Geryuganshoop asking him to return to the ship to deal with Saitama but he is preoccupied with the S-Class heroes, Metal Bat then discovers his core which is his weakness and proceeds to smash it, taking out one of the heads.
Saitama is exploring the ship when he hears Geryuganshoop voice via telepathy who tries the direct him out of the ship, Saitama follows the opposite of his orders and ends up confronting Boros in his chambers. Geryuganshoop then appears and attempts to fight Saitama using telekinesis.
Geryuganshoop enters a fight with Saitama, and hurls a rock at him with his telekinesis. Unaffected, Saitama retaliates by throwing a rock back at him with enough force to split his head in two. Boros who has observed the fight is impressed and explains how he has traveled through the entire universe, because a prophet who told him about someone who could give him an enjoyable fight. Saitama then punches Boros in the stomach scolding him about rampaging on other planets. Boros miraculously survives, but his armor that seals his strength is broken, Boros then begins a new transformation.
The Dark Matter Gunner focuses the ships cannons in order to support Melzargard against the S-Class heroes, but just when the missiles were about to hit the ground Tatsumaki stops them in mid-air.
Tatsumaki stops the missiles saving the S-Class heroes, she then sends them back at the ship damaging it, to the distress of Melzargard. The S-Class heroes then gain the advantage over Melzargard with Bang finding another one of his cores, Melzargard then hits back at Bang.
The fight between Saitama and Boros continues, with both individuals acknowledging each other's strength.
Tanktop Master then tries to help Tatsumaki by throwing a piece of a building at the ship, but ends up getting caught up in her own psychic attack against the ship. The crew of the ship worry about their fate due to the attack. Genos, Drive Knight and Child Emperor watch as Tatsumaki attacks the ship. Drive Knight decides that he's not needed and leaves, but not before giving a warning to Genos about Metal Knight.
Melzargard grows to a larger size, gloating about how he has defeated Bang and threatening the other S-Class heroes. Bang proceeds to get up, looking unhurt and shocking Melzargard, who then is attacked by Atomic Samurai and finally defeated by Bang.
Saitama and Boros continue their fight, with Boros beginning to gain an upper hand firing a laser from his chest and then knocking Saitama down from behind. Boros explains about his home planet being almost inhospitable and how they have adapted regenerative powers to survive. Saitama seems to ignore Boros's story asking Boros if he is done with the fight.
Same as version 1. Except instead of Boros kicking Saitama in the back while in the air sending him crashing into the ship, Saitama comes out of the rubble while Boros appears behind and hitting him in the back of the head.
Boros seemingly believes that Saitama took damage from his attack. He boastfully explains to Saitama about his regenerative capabilities, remarking on how eventually Saitama will get tired and lose. Saitama simply tells him to shut up, asking him if Boros was done with the fight, only to have Boros get mad and unleash his Meteoric Burst on attack Saitama. Due to his powerful kick, Saitama was blown away towards the moon.
Saitama realized that Boros gave his best, so he decided to take the match a little serious. Saitama jumped back to the ship from the moon and, due to powerful impact, the ships take a critical damage. Saitama told Boros to give him all he's got. Boros accepts his offer and then he jumps far away from him. He released his powerful trump card, Planet Buster Roar Cannon, in short a powerful blast through him. With the powerful blast coming toward him, Saitama decided to use his trump card against it, the Serious Series: Serious Strike.
Therefore, Saitama finally defeated Lord Boros. While slowly dying, Lord Boros told him that he cheated during the fight because he never used his full power against him. Saitama replied that he gave his best too but not half yet. Then Lord Boros died in vain because the prophecy was wrong as Saitama was too strong.
The spaceship crashes into the earth forcing all of the S-Class heroes to escape and completely leaving A-City in ruins. Sweet Mask appears to demand a reason for A-Class state, when the S-Class heroes explain that they dealt with the situation Sweet Mask explains that they should have done a better job, this leads to an altercation between Metal Bat and Sweet Mask. Suddenly Metal Knight appears, to the surprise of the heroes, when Genos questions his appearance, it is revealed he has appeared with the intention of using the alien technology for weapons that are needed "for the sake of peace".
Superalloy Darkshine appears finding alien survivors from the crash, but they are slaughtered by Sweet Mask to everyone's surprise. He explains that he killed then as they had no right to live because they are evil and he only executed them on the spot.
Saitama appears from the wreckage of the ship surprising both Tatsumaki and Superalloy Darkshine. Genos goes to check on Saitama who explains about his fight with Boros, ignoring Tatsumaki. Tatsumaki who is angered about being ignored starts to hurl insults at Saitama, Genos responds by calling Tatsumaki a "Shitty Brat", who throws him into a building via telekinesis. Bang calms down Tatsumaki, saying that she must be more conscious of her rank.
The Hero Association HQ was rebuilt as an Iron Fortress. The spaceship was retrieved by Metal Knight and moved elsewhere.
- Saitama is promoted to rank 63 in B-Class.
- Sitch reveals a prophecy that the world will face destruction in six months.
- A-City is demolished by the Dark Matter Theives.