In her first year of marriage, Airi was still having daily sex with her husband and living happily in love. However, suddenly her husband got assigned a one-month long business trip. To ease his loneliness during the trip, he decided to have his sister stay with Airi. Little did they know that Airi's sister Sumire had been secretly eyeing Airi since she started dating her brother and was hoping to draw her into the world of lesbians. Note that the contents may vary depending on the recording method used.