Manyû sanshimai: Ire kui rankô

Original title: 魔乳三姉妹　入れ喰い乱交

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.