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.

Pretty Young Sister follows the journey of a woman as she explores her sexuality, navigates the complexities of relationships, and confronts societal taboos. The film delves into themes of desire, intimacy, and the blurred lines between love and lust.

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...

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.

Jeong-woo who is tired of instant food and living alone decides to order delivery food. However, Ga-yeong, the delivery driver, is unfriendly even if she was late. To criticize Ga-yeong sarcastically, he asks, "Will you do everything if I give you money?" Ga-yeong takes the words seriously and attacks Jeong-woo's house. Woo-hyeon is questioning the woman's underwear found at the house of Hyeon-min who came to play at Jeong-woo's house, and learns the situation through Ga-yeong who just came. He said he was better than Jeong-woo. However, Ga-yeong is already full of thoughts of eating both of them.