Da-hyeon and Hyeon-soo are sisters who grew up close to each other without their parents and still live together even after Da-hyeon got married. Sex isn't good between Da-hyeon and her husband, so she fucks her husband's stepfather when he comes to visit. Meanwhile, her husband is also fucking her little sister Hyeon-soo. And when her husband's stepfather and her little sister get to be alone in their house one day, they also make love. Da-hyeon finds out about Hyeon-soo's affair with her husband first, and then her affair with her husband's stepfather second. What is she to do? Will she confront her slutty little sister and destroy the beautiful relationship they have which has stood the test of time? Or will she let bygones be bygones and just have a foursome with her and with the two men closest to them?