Natsuo Fujii is in love with his teacher, Hina. Attempting to forget his feelings towards her, Natsuo goes to a mixer with his classmates where he meets an odd girl named Rui Tachibana. In a strange turn of events, Rui asks Natsuo to sneak out with her and do her a favor. To his surprise, their destination is Rui's house—and her request is for him to have sex with her. There's no love behind the act; she just wants to learn from the experience. Thinking that it might help him forget about Hina, Natsuo hesitantly agrees. After this unusual encounter Natsuo now faces a new problem. With his father remarrying, he ends up with a new pair of stepsisters; unfortunately, he knows these two girls all too well. He soon finds out his new siblings are none other than Hina and Rui! Now living with bo