Sang-woo's daily life changes 180 degrees ever since his sister's friend Jeong-hee comes to live in their house. Everytime he sees her walking around the house with wet hair and wearing just a big T-shirt, he gets erotic thoughts. He dreams of having sex with her just once until one day, he does. However, since then, Jeong-hee can't forget the night with Sang-woo and how much better he was than her fiance and starts coming back to his room...

My Sister's Friend 2

Min Jung's husband has been traveling for 300 days and she feels lonely. When her friend Chung Hee moves in next door, Min Jung often visits her house, during which she keeps meeting Jae Ho, Jung Hee's son. After having an impulsive affair with Jae Ho, Min Jung tries to forget about it, saying, "It was a mistake," but she keeps remembering his young body.