The story starts with Keita Suminoe, a male third-year junior high student, studying for his high school entrance exams. He lives with his older twin stepsisters, Ako and Riko, who are not related to him by blood. They help him study for his exams. Even though he doesn't like himself for it at first, he starts to feel attracted to his two step-sisters, and his two parents encourage him to marry one of them.