A ditsy and unpopular Oh Ha-Ni (Jung So-Min) is in love with her opposite, Baek Seung-Jo (Kim Hyun-Joong) who is a smart and popular boy. Tension rises when Oh Ha-Ni writes and delivers her love letter to Baek Seung-Jo to which he gives back, graded.
Embarrassed and heart-broken, Oh Ha-Ni goes home only to have her house collapse in on her, her father and her three friends. As fate would have it, Oh Ha-Ni and her father – Oh Ki-Dong (Kang Nam-Gil) end up being invited to live with Baek Seung-Jo and his family.
Oh Ha-Ni, who is still devoted to Baek Seung-Jo works hard to try and impress Seung-Jo.
Along the way,Ha-Ni becomes tired of Seung-Jo’s cold and uncaring attitude. Bong Joon-gu, who has been liking Ha-Ni for a long time, began chasing for her affection and ultimately, her hand for marriage.
Seung-Jo then realises that she is about to slip away and starts chasing her to win her back.