natsuo fujii is hopelessly in love with his teacher, hina. trying to move on, he agrees to a mixer. there he meets an odd girl, rui tachibana, who invites him to sneak out. she takes him to her house and asks him to have sex with her. natsuo, frustrated that his love will not bear fruit anyway, says goodbye to his virginity. the next day, natsuo's dad tells him that he wants to remarry and his prospective partner is coming to their house that evening. when the door opens, it turns out that rui is hina's younger sister and both are daughters of the woman his father wants to marry, tsukiko tachibana.