The Korean movie 18+ Young Sister In Law revolves around the dangerous cohabitation of three men and women of the same age. Byeong-soo, a student on leave who luckily won a script contest while still in school and had to write a script for a movie, moves into the home of a senior student who already has a family while looking for a quiet place to live, focusing on creative work away from the noisy city.
However, Byeong-soo is not the only one who wants to have a peaceful life. Ji-eun (Sihyeon), a sexy young sister-in-law of the same age, and Bo-yeong (Jena), a sexy young girl, live with her junior Byeong-soo instead of her young husband. The cohabitation of three men and women for a month is unsolvable, and it is almost a crisis. From a peaceful life, they build relationships and life encounters many troubles.