House of Evolution Arc

Standing as the second arc of the series and the second of the Introduction Saga, this installment brings the cyborg Genos into Saitama's life and establishes the House of Evolution, a covert lab breeding artificial monsters. It marks the first time an enemy senses just how abnormal Saitama truly is, and the first time he explains the deceptively simple regimen behind his power.

A mosquito swarm engulfs Z-City under the command of Mosquito Girl, and Genos arrives to destroy her. When the fight turns against him and she tears into his body, Saitama swats her aside and saves the cyborg, who immediately begs to become his pupil. Saitama grudgingly accepts. Soon the laboratory's creations come hunting: Kamakyuri crashes through the ceiling and is dispatched instantly, and the duo splits up to handle Frog-Man, Slugerous, Beast King, Ground Dragon, and Armored Gorilla, the last of whom Genos interrogates for the group's location.

Acting on that intelligence, the two reach the House of Evolution's headquarters, where Genos is blindsided by the compound's strongest experiment, Carnage Kabuto. The beast challenges Saitama, but Genos insists on fighting first and is soundly beaten. As Saitama steps in, the lab's founder, Dr. Genus, is certain the newcomer stands no chance. Carnage Kabuto's instincts warn him against striking, and when Saitama recounts the ordinary training that made him strong, no one believes a word of it.

The disbelief drives Carnage Kabuto into his rampaging Carnage Mode, unleashing a storm of attacks that leave Saitama completely unharmed. Suddenly remembering a supermarket sale, Saitama ends the creature with one punch, leaving both Genos and Dr. Genus speechless. The encounter makes Carnage Kabuto the first monster to grasp Saitama's true nature, and the defeat prompts Genus to abandon his ambitions and settle, along with Armored Gorilla, into a quiet civilian life. Genos, now formally Saitama's disciple, has shared the tragic backstory driving his own crusade.