A clumsy high school girl named Oh Ha-ni (Jung So-min) who is at the bottom of her class has had a crush on a popular and genius male student called Baek Seung-jo (Kim Hyun-joong) ever since she laid eyes on him on the first day of high school. Ha-ni decides to confess through a letter, but is rudely rejected by Seung-jo, who tells her that he hates stupid girls and returns the letter, having corrected all of her grammatical and spelling mistakes and giving her a mark of D-.  Fate intervenes when an earthquake ruins Ha-ni's family's newly built home. While the house gets rebuilt, Ha-ni and her father are invited to stay at the home of her dad Oh Ki-dong's (Kang Nam-gil) childhood friend, who happens to be Seung-jo's father Baek Soo-chang (Oh Kyung-soo). Ha-ni's feelings for Seung-jo are rekindled, and she is more than determined to impress him. Seung-jo remains insensitive to her feelings and finds her a complete nuisance. However, Seung-jo's matchmaker mom, Hwang Geum-hee (Jung Hye-young) helps Ha-ni and becomes determined to get the two together.  During her time with Seung-jo and his family, Ha-ni blackmails Seung-jo into tutoring her to become part of the top fifty ranked students in their high school. Seung-jo reluctantly agrees and over the weeks as they tutor together, Ha-ni's feelings grow continually stronger for Seung-jo, and his feelings for her also strengthen though he does not show it and is still very confused by them. When the results of the ranking come out, Seung-jo tells her to look at her placing and she is shocked to find that she ranked #50. Their high school year is filled with heartbreak, confusion, anger. Ha-ni's unrequited love remains but slowly and gradually, Seung-jo comes to like Ha-ni and finds her company very enjoyable. After several dramatic events involving Ha-ni, Seung-jo grows closer still to her. When Ha-ni says she will forget him and no longer be interested in him in front of the senior year, he panics a little and strings her along by kissing her on their graduation night to renew her feelings for him and announces that he has decided to attend the same University as Ha-ni.  Seung-jo and Ha-ni soon graduate high school and move on to university. There, Seung-jo meets Yoon Hae-ra (Lee Si-young), the female equivalent of Baek Seung-jo. While Ha-ni's classmate Bong Joon-gu (Lee Tae-sung) still harbours, despite many incidents that have continuously broke his heart, feelings for Ha-ni. However, Seung-jo is still the one Ha-ni likes, despite the fact that he is always with Hae-ra and has stated that he lives to tease her. Hae-ra falls in love with Seung-jo and they begin working together at a restaurant, which makes Ha-ni jealous.  Ha-ni's feelings continue to grow just as the friendship between Seung-jo and Hae-ra does too. When Seung-jo's father becomes hospitalized, Seung-jo takes over his father's gaming company temporarily. He is introduced to the investor's granddaughter at an arranged marriage meeting, who turns out to be none other than Hae-ra. Joon-gu doesn't give up on trying to win Ha-ni's heart, and one day, he takes his infatuation one step further by proposing to Ha-ni. However, Ha-ni rejects Joon-gu and exits her father's noodle shop in guilt. It's raining and she is surprised to find Seung-jo standing there with an umbrella, waiting for her. Seung-jo claims to be curious about her answer for Joon-gu, knowing that he had proposed, but Ha-ni refuses to tell him. This leads to an argument between the two, causing Ha-ni to have an outburst, which is interrupted when Seung-jo suddenly kisses her, finally revealing his true feelings.  Seung-jo announces to all family members that he and Ha-ni will get married once they graduate, to the excitement and happiness of their family members, especially Seung-jo's mother. Seung-jo loosens up and embraces his feelings for Ha-ni and Joon-gu is heartbroken. Seung-jo tells Hae-ra to which she pretends to be happy with his decision and tells him that she is happy that finally Seung-jo is able to know what he actually wants. Hae-ra releases her heartache through playing more than four hours of tennis in frustration while Kyung-soo (Choi Sung-gook), her crusher waits and watches her until she finishes. Seung-jo's mother announces their marriage date which is within one week. However, in the time period leading up to the wedding, arguments break out between Ha-ni and Seung-jo. But despite Seung-jo's temporary uncertainty of marriage, they marry.  During their honeymoon, they encounter another couple. The woman flirts with Seung-jo in front of Ha-ni, and takes up all of their days of their honeymoon by tagging along with them, to Ha-ni's annoyance. She fakes an injury so that he will inspect her (Seung-jo mentioned that he is studying medicine). Ha-ni gets jealous, and she and Seung-jo argue. After Ha-ni leaves the room, the woman tries to seduce Seung-jo into bed, but Seung-jo, realising she was putting up a façade, rejects her and tells her to 'never compare herself to Ha-ni'. Then Seung-jo goes searching for Ha-ni and the two reconcile after he tickles her.  Chris, a British-Korean girl, enters their lives. She speaks Korean fluently and develops a major and conspicuous crush on Joon-gu. But Joon-gu rejects her saying he only has room for Ha-ni in his heart; it is later revealed he was in denial, and has begun to have feelings for Chris.  Ha-ni tries to switch her major to nursing to be closer to Seung-jo, but her grades aren't sufficient. On her way to a date with Seung-jo on Christmas Eve, she witnesses an accident. Despite the risk of arrest if it's done incorrectly, Ha-ni successfully performs CPR on the person and accompanies her to the hospital. It makes her late for their date, but she finds Seung-jo waiting for her. Meanwhile, Min-ah becomes a famous cartoon artist, having drawn their high school lives including Ha-ni's confession to Seung-jo, and it hints at a subtle relationship between her and another man at one of her signing events. Joo-ri shares an unspoken relationship with a guy who visits her salon everyday for a trim just to see her and they reunite after his army training of two years. Eun-jo (Choi Won-hong) deals with the same situation as Seung-jo, as a girl confesses to him. Seung-jo's mother, father and Ha-ni's father are seen holidaying and drinking a beverage while resting on lounging chairs. The finale ends with Ha-ni and Seung-jo making out in the company car.