Shinhwa Group is shown to be one of South Korea's largest conglomerates and is headed by the strictly arrogant Kang Hee-soo. Her son Gu Jun-pyo (Lee Min-ho) is the leader of F4, the most popular and powerful group of boys at the prestigious Shinhwa High School. Yoon Ji-hu (Kim Hyun-Joong), So Yi-jung, and Song Woo-bin – Gu Jun-pyo's friends and rich heirs in their own right – are all members of the group. When F4 bullies a boy to the verge of suicide, the poor but vivacious Geum Jan-di (Ku Hye-sun) saves his life and receives a swimming scholarship to attend the High School. Geum Jan-di's family runs a dry cleaning business and she lives in a small home with her parents, Geum Il-bong and Na Gong-joo, and her younger brother Geum Kang-san.
Immediately, Geum Jan-di dislikes her wealthy classmates and is disgusted by the obsession surrounding the F4 boys. Irked by her strong-headed nature, Gu Jun-pyo singles out Geum Jan-di to bully but she stands up to him and he begins to fall in love with her. Meanwhile, Geum Jan-di develops feelings for Yoon Ji-hu. She soon discovers that Yoon Ji-hu fosters a one-sided love for Min Seo-hyun, a successful model and his childhood friend.
After Yoon Ji-hu leaves the country to pursue a relationship with Min Seo-hyun, Geum Jan-di begins to grudgingly spend more time with Gu Jun-pyo. He declares to the school that she is his girlfriend. Shortly afterward, Geum Jan-di's friend Oh Min-ji drugs her at a party and takes pictures of her lying unconscious in bed with a guy. When the photos are leaked in the school, Gu Jun-pyo is outraged and believes that Geum Jan-di has been cheating on him. Subsequently, his fans and crowd of adoring classmates begin to bully and abuse Geum Jan-di. Oh Min-ji secretly discloses to Gu Jun-pyo of her involvement with the photos. It is revealed that they have been classmates since kindergarten, and that Oh Min-ji has long been trying to win his attention by undergoing cosmetic plastic surgery. Gu Jun-pyo shuns Oh Min-ji for betraying her friend and rushes to save Geum Jan-di, who is attacked by bullies. He apologizes for doubting her and their bond grows stronger.
Yoon Ji-hu returns, causing Geum Jan-di to feel confused about her feelings. While Gu Jun-pyo relentlessly tries to shower her with affection, she is increasingly distracted by Yoon Ji-hu. During a weekend trip on a private island, Gu Jun-pyo walks in on her sharing a kiss with Yoon Ji-hu and he expresses his hurt in a rage, and punches Yoon Ji-hu in distress. The next morning, he leaves the island and becomes reclusive, causing Geum Jan-di to feel guilty. She realizes that she is in love with him and declares this when Gu Jun-pyo feigns an accident. In the meantime her best friend, Chu Ga-eul, bonds with F4 member So Yi-jung and she develops a crush on him.
When Kang Hee-soo learns of Gu Jun-pyo and Geum Jan-di's relationship, she executes numerous attempts to humiliate Geum Jan-di and her family due to their social status. Geum Jan-di perseveres and finds a friend in an overly-helpful model, who helps her earn money by modeling for a magazine. This angers Gu Jun-pyo as he feels hurt by Geum Jan-di seeking help from someone else. When she is kidnapped by the model, it is revealed that he is the younger brother of the student whose suicide Geum Jan-di had prevented, and he seeks revenge. Gu Jun-pyo arrives alone and the model has him beat up in front of a tied up Geum Jan-di, but they are rescued by the F4 members in time. However, in this incident Geum Jan-di hurts her shoulder, which prevents her from swimming in the future.
After Gu Jun-pyo's father falls ill, his mother forces him to begin training to lead Shinhwa. He leaves the city, turns cold toward his friends and Geum Jan-di, and begins to accompany his mother for formal arrangements. After 6 months, Geum Jan-di and F4 pursue him and try to convince him to break free, however he turns them away. Kang Hee-soo plots to keep the distance between them and arranges Gu Jun-pyo's engagement with the heiress Ha Jae-kyung. Geum Jan-di approaches Gu Jun-hee, Gu Jun-pyo's loving older sister, to help her find work and she becomes Gu Jun-pyo's personal maid. It is evident that both Gu Jun-pyo and Geum Jan-di still love each other, however Ha Jae-kyung falls in love with Gu Jun-pyo and tries to win him over. She soon realizes that it is futile, and during the procession of their wedding, she refuses to marry Gu Jun-pyo and expresses that she will feel guilty that she was standing in the way for a meant to be relationship.
Geum Jan-di and Gu Jun-pyo reunite, however due to her friends being threatened by his mother, Geum Jan-di leaves the city after spending loving moments with him. Gu Jun-pyo is heartbroken and gets into trouble with the police, later that week he meets with a car accident while trying to save Yoon Ji-hu's life that causes him to lose his memory of Geum Jan-di. Encouraged by her mother to pursue her heart, Geum Jan-di returns but is saddened that Gu Jun-pyo cannot recall who she is. After asking him to try to remember her name, she falls into a pool as he watches, causing him to be flooded by memories of them saving each other from drowning. He calls out her name and rescues her and they unite once again. Although he asks her to marry him, they decide it is best to pursue their individual dreams first. Gu Jun-pyo leaves for America, where he furthers the Shinhwa group and becomes a successful businessman.
Four years later, it is shown that Yoon Ji-hu is graduating medical school and Geum Jan-di is still a third-year medical student, while So Yi-jung is a successful architect and returns from Sweden for Chu Ga-eul, who has become a teacher. Gu Jun-pyo arrives in a helicopter and proposes to Geum Jan-di at the beach side; the rest of the F4 arrive at the same time and they all walk alongside the beach, smiling happily.
Nice text