Yui is the capable older sister who handles the housework, while Karin is quiet and easily led. They live comfortably with their wealthy stepfather after their mother remarries, but everything changes when she is hospitalized for a chronic illness. The once-kind man exposes his lust and targets both adult sisters, sweet-talking compliant Karin into sex while forcibly breeding defiant Yui. Exploiting their different personalities, he takes control of their bodies and minds until they compete to serve him sexually. Thoroughly brainwashed, they can think only of becoming their stepfather’s breeding vessels and beg to be chosen first.