Because their father passed away early and their mother remarried, the two sisters Miru and Shion have a new family. Miru is the older sister, a lively, intelligent, and innocent high school girl. Shion is the younger sister, quiet and somewhat resentful towards their new stepfather. Some time later, their mother becomes pregnant. The stepfather takes both of them to their grandparents' house for the convenience of childbirth. With their mother not at home, the stepfather always looks at both sisters with lustful eyes. And the first person he attacks is Shion. Taking advantage of Shion's insecurity compared to her sister, he quickly manipulates her emotions, making her obey his every word. Miru discovers this and, in order to save Shion, willingly becomes a substitute. But the pleasure that the stepfather brings gradually causes her to lose herself. Both sisters are slowly brainwashed by him, unable to live without his cock...