Pretty Young Sister 4

Ji-yeong and Woo-hyeon is on a fun, casual relationship. However, Woo-hyeon finds out that his friend Min-soo likes Ji-yeong and decides to be a stepping stone between them. Will Min-soo and Ji-yeong be able to date after many twists and turns? Or will Ji-yeong go back to Woo-hyeon's arms? The story of two men surrouding Ji-yeong's heart begins.

Dong-hoon and Min-soo, who are college friends, are best friends with their boyish female friend Ji-hyeon from the same department. Min-soo secretly likes this kind of Ji-hyeon. One day, Ji-hyeon, Min-soo and Dong-hoon drink casually, and Ji-hyeon makes a mistake. Ji-hyeon, who drank a lot the day before, seems to have slept with someone, but she can't remember whether it was Dong-hoon or Min-soo. But Ji-hyeon is more confused by the behavior of two men who are so indifferent... Was it Min-soo who spent the night with Jihyeon? Was it Dong-hoon? The sweet, gruesome love story that falls into an unknown mystery is just beginning.

Newly-moved Yoo-jeong had a housewarming party. So-hyeon and Min-soo gather their friends at the housewarming party and meet for the first time. Yoo-jeong falls asleep alone, and Min-soo secretly kisses Yoo-jeong. So-hyeon, who finds out, threatens Min-soo and plays with him. Min-soo had no choice but to accept So-hyeon's terms. Yoo-jeong misunderstands them. But in the process, Yoo-jeong herself finds herself interested in Min-soo. Recognizing that, So-hyeon gives up the room he originally intended to live in to Min-soo and leaves.

Yeon-joo feels good about Min-soo who found his cellphone, saying that it is fate. Although they are dating, she is disappointed in Min-soo. While kissing at Min-soo's house, roommate Jae-kyeong catches them. Yeon-joo is hurt when Min-soo had to hide her from Jae-kyeong. Jae-kyeong comforts Yeon-joo and begins a new fate. And the truth that Jae-kyeong is the person who found the cellphone has been revealed...