Irina, the third daughter of the three Nemoto sisters, is a plain, college science student who wears a white coat and glasses at school. It seems that things are going well with her boyfriend Soga. One day, while Irina is studying ancient Japanese history, she discovers a system in which women share a man sexually.
| Actor | Role |
|---|---|
| AAkika KitagawaIrina Nemoto | Irina Nemoto |
| AAnna TomoMasami Nemoto | Masami Nemoto |
| KKyôko KazamaOriha Nemoto | Oriha Nemoto |
 Mutsuo YoshiokaSokuten Soga | Sokuten Soga |
 Seiji NakamitsuToshihiko Mitsukawa | Toshihiko Mitsukawa |
| NNaohiro HirakawaGinichi Kuwai | Ginichi Kuwai |