The Garou Introduction Arc is the tenth arc of the series, and the first arc of the Human Monster Saga.
Genos returns to Dr. Kuseno with parts from G4, asking Dr. Kuseno to use them to make him stronger, to which the doctor is amazed and eventually agrees. However Dr. Kuseno is still worried for Genos and asks him to not go too far, to which Genos replies that he is not and it is still within his limit, even telling the doctor the goal Saitama set for him. Worrying for Genos' safety the doctor asks to meet with Saitama. The doctor then checks to see if Genos is still after is original goal to finding the cyborg that destroyed his hometown, to which Genos replies he is but has not yet found a clue to the Cyborg's whereabouts, but shows he is still determined as ever to destroy said "Evil". Dr. Kuseno sees Genos' high-strung attitude and tells him he is concerned about him, telling him to not act rash and call him before confronting the "Evil Cyborg", telling him he is not alone in that fight, to which Genos agrees.
Meanwhile at the Hero Association Headquarters. Sitch had gathered all villains and criminals in the headquarter, telling them it is not a trap and is not about their criminal activities. He even tells them to ignore the three heroes behind him (A-Class Rank 5 Heavy Tank Fundoshi, A-Class Rank 6 Blue Fire and A-Class Rank 7 Magic Trick Man) as they are just his bodyguards, which causes the criminals to get a bit ambitious. The heroes are concerned of what reason could be for said gathering of criminals, but are still bracing themselves, while Sitch knowing the strength of said heroes, thinking it would be foolish to attack. Just when Sitch was about to hand out papers to the criminals, a voice is heard, asking of the whereabouts of the hero called Saitama, thinking he lives in said facility. Hearing Saitama is not there, the "voice" claims he has no reason to stay, which irritates Blue Fire. Said "voice" is revealed to be Speed-o'-Sound Sonic who throws all pamphlets in the air, much to Sitch's surprise, for he had not handed them out yet. Sonic talks about the reason for said gathering, which was to bring all help possible to protect the earth, to which he refuses, claiming he is not interested.
Sonic is leaving the meeting. After hearing of the real reason for the gathering, Blue Fire is angered and confronts Sitch, asking him if it was true, to which Sitch tells him it was, since it is a dire time and everyone must now work together to save the earth. Sitch then proceeds to talk of the number of heroes currently in the hero association, and claims them not to be enough. Sitch says that because of the criminals strength that can rival some heroes, they are there, and that if they help they will be rewarded. Fundoshi mocks the idea, which provokes one of the criminals, who taunts him, Fundoshi then pummels the criminal to the ground. A man is shouting, and talks about the prophecy, claiming it is about him, provoking everyone to attack him, he reveals himself to be Garou.
Garou is excitingly yelling at the other criminals to fight, since it's the only reason he came, to see who is the strongest. Sitch rejected such an idea and tells Garou to leave the facility. Garou later changes his mind and decided to conduct evil. Sitch advises his bodyguards to deal with him to which they agree.
Garou then reminiscence his days as a child, where he watched a show about Justice Man and was rooting for the villains. He later found out that justice always prevails and decided he will become the strongest monster ever to change that scenario, while choking Magic Trick Man.
Heavy Tank Loincloth attacks Garou in an attempt to save Magic Trick Man, but Garou manages to break his arm and knocks him out with a single punch. The criminals cheer on Garou, only for him to state they will die too. As the criminals defend themselves, stating they are not on the heroes side, Garou simply replies he is neither on the heroes nor human side, instead on the monsters side. Blue Fire agrees with that statement and tries to burn Garou with one of his attack only for Garou to rip off his arm. While Sitch calls for backup, the criminals attack Garou.
Outside, Sonic is leaving the Hero Association Headquarters. He heard commotion coming from the headquarters, but simply brushes it off, since he already wasted his time, for not finding Saitama and declaring he will one day defeat him. In the mean time Saitama is playing a game with King, who complains of always losing.
At the headquarters, Garou managed to beat every single criminal and declared a war against the heroes. He declares in half a year, he will become stronger and challenge King, the strongest man on earth, leaving Sitch covering in the corner.
- Garou officially starts his hero hunt.
- Blue Fire loses one of his arms to Garou.