The drama tells the story of the youth love story of a bright and lively high school girl Shin Song-yi who has been secretly in love with her childhood sweetheart Cha Heon for 17 years. Kim Yo-han plays the role of Cha Heon, a 17-year-old high school student who has been secretly in love with Shin Sol-yi for 17 years. He is a handsome man with a beautiful face and extremely smart. So Joo-yeon plays the role of Shin Sol-yi, a 17-year-old high school girl who has been secretly in love with Cha Heon for 17 years. She is lively and cheerful, very proactive, and a "straightforward girl" who confesses directly to the person she likes.