A monster is moving toward City Z, one that has been created by the mysterious organization known as The House of Evolution. Genos, a young cyborg, attempts to stop it, but gets in over his head before Saitama happens along.

Saitama and Genos glean information about The House of Evolution and its founder Dr. Genus, a scientist with dangerous ideas on artificial human evolution. Meanwhile, Dr. Genus learns of Saitama’s amazing strength and has plans of his own for One-Punch Man.