Joon-soo learns that his friend Hee-gyu's sister Hee-jin has no where to go after being scammed, so he suggests living with him. The two start living together. They are close like brother and sister but they can never be that. The two start getting funny and Joon-soo can't go to sleep at the thought of Hee-jin sleeping just a wall away from him. Mi-na, Joon-soo's girlfriend notices Joon-soo changing and seduces Joon-soo at his house where Hee-jin is. Is it the girlfriend or the sister?

My Friend's Older Sister

Twenty year-old Ki-chan lives alone with his father. One day Yoon-seo (Kim Jeong-ah), his stepmother comes to live with them. She tries to be friendly with Ki-chan but he appraoches her as a woman and starts showing her that. Yoon-seo strongly rejects him but he knows that she wants him too. Their relationship turns into something dangerous. Then one day, Min-jeong, who has had a crush on Ki-chan for a long time, finds out what is going on between the two. The young stepmother fools her husband and falls in love with his son and the son falls in love with his father's woman. They make a deal with Min-jeong to make sure she doesn't say anything.