Swingers Party with My Friend.

Sin-wook and Dong-soo have been friends since they were kids. Sin-wook and his wife swap partners with Dong-soo and his wife. They start playing Strip Go Stop and the game gets serious. The four of them have sex, but they end up falling for each others' wives and husbands and even meet in secret. One day, when their sex life reaches its peak, Sin-wook's wife, Yeon-hee, gets pregnant, not knowing whose baby it is. Her relationship with Sin-wook is on thin ice and then the baby is born. They do a DNA test to find out who the father is.