The House of Evolution Arc is the second arc of the series, and the second arc of the Introduction Saga.
While at his home, Saitama thinks about how his emotions are slowly dulling because of his overwhelming power, when a mosquito starts to annoy him. Saitama then finds out on the news that a mosquito outbreak is ongoing in Z-City.
Outside, Mosquito Girl is controlling masses of mosquitos and wreaking havoc. Genos arrives, saying that he will eliminate her. As Mosquito Girl is about to transform, Saitama rushes into the battlefield to interrupt their fight. Genos starts struggling immensely, as Mosquito Girl is ripping off parts of his cyborg body, but Saitama slaps her away, killing her and saving Genos in the process. After seeing Saitama's strength, Genos begs Saitama to make him his disciple. Saitama reluctantly agrees.
Later, Genos visits Saitama's apartment and shares his tragic backstory with him and begs Saitama to teach him how to become strong. Suddenly, Kamakyuri bursts through the roof, only to get instantly whacked by Saitama. Saitama and Genos go outside and deal with Frog Man and Slugerous, but they are split up when Genos is forced to deal with Armored Gorilla. Saitama kills Beast King and Ground Dragon and Genos starts interrogating Armored Gorilla after defeating him.
Using the info they got from Armored Gorilla, Saitama and Genos arrive 4 hours later at the House of Evolution HQ. Genos blows up his surroundings to find an entrance. While walking through a hall, Genos gets smashed by the House of Evolution's strongest specimen, Carnage Kabuto. Carnage Kabuto challenges Saitama to a fight, but just before it is about to start, Genos returns and fights Carnage Kabuto instead. However, he is utterly defeated. Saitama then prepares to take on Carnage Kabuto, while Dr. Genus, the leader of the House of Evolution, thinks to himself that Saitama doesn't stand a chance. Carnage Kabuto attacks first, but hesitates, thinking that if he completed his move, he would have gotten killed. Saitama, after getting asked by Carnage Kabuto, explains to everybody how he got so incredibly strong, but nobody believes him, including Genos. This only angers Carnage Kabuto, as he enters Carnage Mode and unleashes a barrage of different attacks onto an unaffected Saitama. Saitama realizes he is about to miss a sale, and kills Carnage Kabuto in one punch so he can make it in time. Dr. Genus and Genos are stunned.
- Genos is introduced and becomes Saitama's disciple.
- Genos' backstory is revealed.
- Carnage Kabuto is the first monster to sense Saitama's true power.
- Saitama reveals for the first time how he trained to become so incredibly powerful.
- Saitama and Genos dismantle the House of Evolution.
- Dr. Genus and Armored Gorilla start living normal civilian lives.