Da-hyeon and Hyeon-soo grew up like sisters without their parents, and still live together even after Da-hyeon got married. Sex isn't good between Da-hyeon and her husband, so when her husband is out, she has an affair with a stranger. One day, Hyeon-soo's step-father, Seok-hyeon, goes to his son's house and sees his son's wife's sister there all by herself...