18-year-old Ryouko Murakami lost her parents in a sudden accident and was living in solitude until she was adopted into the Hojo family, where she finally began to feel the warmth of family life. Her adoptive parents were kind, and her stepsister Maki Hojo treated her like a real younger sister. One day, shortly after Ryouko moved out to live on her own, she received an unexpected phone call from Maki, leaving her filled with shock and unease. That night, Maki came over to stay, but her demeanor was completely different—cold and distant. Without warning, she began touching Ryouko's body. From that moment on, everything changed.