Shin-wook and Dongsu have been friends since childhood. Shin-wook, who feels boredom, is swapping with a fellow initiate. Starting with the GoStop game, the couple's swapping gets deeper. The four came together to have sex, but they fell in love with their partner. One day when their sex life climaxed , Shin Wook's wife, Yeon Hee, became pregnant. Unable to know who their father is, the conflict deepens. Their relationship is ruined. A few months later, when they were born, who did the paternity test, who is the father?