In an ordinary-looking house, live the owner couple, Se-kyung and Jung-sik, their daughter,Ma-ri, and Jin-seong who's new to society. The couple is on vacation, and Ma-ri and Jin-seong are alone in the house. One ordinary day, An Uninvited Guest called Soo-jeong comes along. She had a fight with her husband and had come to stay in her brother's house for a few days. However, one night Soo-jeong gets drunk and mistakes Jin-seong's room for hers. As a result, they end up sleeping together. Ma-ri sees Soo-jeong come out of Jin-seong's room, but suspects nothing else. Vegetarian Jin-seong craves sex and sleeps with Soo-jeong once again. However, Soo-jeong goes back to her husband, and Jin-seong can't help but feel lonely. Meanwhile, Ma-ri likes Jin-seong and seduces him to have sex with her, but she's outraged when he tells her he had sex with her aunt. This is the story of the ordinary but not so ordinary, secretive house.