My Sister's Friend 2.

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...