The daily life of high-school student Natsuo is heavily complicated after his single father marries the mother of his teacher crush Hina Tachibana and her sister Rui - who he had shared his first experience with with at a preceding mixer.

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.