Based on the movie 'To all the boys I've loved before' 

In which Seulgi's little sister, Yeri, sends out her 4 love letters from years ago. One to Jimin from summer school camp in 10th grade, one to Jinyoung, her ex-best friend and now older sisters boyfriend, another to Mino, the adorable boy from her middle school p.e class, who is now one of the schools biggest play boys and lastly to Kai her childhood friend, who now studies in America