yui, the caring elder sister who handles household chores, and karin, the docile younger sister who is easily led. the two were living comfortably with their step-father, a wealthy man who was their mother's second husband. but a nightmare begins while their mother is hospitalized due to her illness. the kind step-father's attitude suddenly changes, and he aggressively pursues the sisters with his lust. the easily led younger sister is skillfully guided into sex. the strong-willed elder sister is forcibly impregnated. taking advantage of their different personalities, he controls both their minds and bodies. before they know it, the two are competing to serve sexually. "our only value is to become our step-father's breeding sack. i'll go first, i'll go first..." they are brainwashed to the point where nothing else comes to mind.