In a world where people have one of three sexes-alpha, beta, or omega-in addition to being male or female, alphas have pushed their way to the top. Omegas are oppressed, their status determined by whether or not they successfully bear alpha children. An omega named Saimon marries into the prolific Hanabusa family, a major conglomerate, but is unable to produce an heir for his alpha husband Seijuro, and is shunned as a "Megaera," a creature that brings ill fortune. One day, Saimon is summoned by the family patriarch Zenjiro and ordered to locate the man's illegitimate son. Saimon finds the boy, Mamiya, who has been making a life for himself on the streets while hiding the fact that he's an omega. Desperate to regain his position as Seijuro's top spouse, Saimon proposes an audacious plan: pass Mamiya off as an alpha heir and try to gain the headship of the Hanabusa family.