Episode 2

As Saitama tends to his cactus, a broadcast warns that an enormous mosquito swarm is bearing down on Z-City. He tries repeatedly to swat a single mosquito and fails every time. In the deserted streets a looter is drained of blood by the insects, which answer to Mosquito Girl. She tangles with Genos, whose cyborg body ignores the swarm, and the two trade severed limbs before she gorges on the blood her swarm has harvested from nearby wildlife. Her power surge turns the duel lopsided, until Saitama sends her flying skyward with one offhand slap. Awestruck, Genos asks to become his disciple, and Saitama agrees.

Days later Genos turns up at Saitama's door, much to his irritation, and delivers a rambling account of his past with the Mad Cyborg that only wears Saitama down. In a hidden laboratory, Dr. Genus reviews Mosquito Girl's defeat and resolves to capture Saitama for study. When the House of Evolution's messenger interrupts the two heroes at the apartment, it is destroyed at once, and Saitama is dropped underground while Genos confronts the cyborg Armored Gorilla. Saitama quickly climbs out, dispatches both foes, and interrogates the surviving cyborg, who panics on realizing his whole team is gone. The episode ends with Dr. Genus entering a room of anxious subordinates who, once they turn around, prove to be clones of himself.

Genos meets Saitama, watches him obliterate the empowered Mosquito Girl, and commits himself as a disciple. The House of Evolution, led by Dr. Genus, marks Saitama for capture and sends operatives who are swiftly wiped out. Saitama and Genos crush Armored Gorilla and the rest of the strike team, and the final reveal shows Dr. Genus surrounded by identical clones of himself.

Titled The Lone Cyborg, this season one episode falls within the House of Evolution Arc and covers chapters 5 through 8. It aired in Japan on October 11, 2015, and in English on July 23, 2016, opening with The Hero and closing with Hoshi Yori Saki ni Mitsukete Ageru. The staff includes director Shin'ichiro Ushijima, screenwriter Satoshihiro Suzuki, and storyboard artist Shingo Natsume. S-Rank heroes Pig God, Superalloy Darkshine, Metal Bat, and Watchdog Man make brief cameos, and Mosquito Girl delivers the closing monologue over the credits.