Younger Sister's Girl.

Sang-Woo's house, Byeongjin, the younger brother of the family, is about to live for a month. Wanjin pretends to see Sang-Woo's lover, Jinju. Sang-woo finds out that they are dating for a while, and Sang-woo is complicated. The difference is Sangwoo, asking Jinju about her relationship with Wanjin. Her younger brother, Wanjin, doesn't even know Sang-woo's situation, and even invites girlfriend Seol-young to Sang-woo's house. While Wanjin goes out, Sang-woo twists Seol-young with revenge on Wan-jin. It does not come in. While Sang-woo and Seol-young are getting deeper between them, Jin-ju is coming home to reconcile with Sang-woo. So, who will she choose between her girlfriend Jin-ju and her sister Seol-young?