The series begins with Geum Jan-di standing up to F4's reign of terror at Shinhwa High after receiving a red card. Her defiance captures Gu Jun-pyo's heart, though she initially falls for Yoon Ji-hoo. Ji-hoo steps back as her guardian angel when he realizes Jun-pyo's genuine feelings.
When Jun-pyo's father collapses into a coma, Jun-pyo moves to Macau to manage Shinhwa Group under his tyrannical mother, Chairwoman Kang. She forces an engagement with Ha Jae-kyung to secure a corporate merger, leading Jun-pyo to treat Jan-di coldly to protect her. Jae-kyung eventually calls off the wedding at the altar upon realizing their bond. In response, Chairwoman Kang bankrupts Jan-di's family, forcing Jan-di to work as a maid in Jun-pyo's mansion. Later, Jan-di suffers a permanent shoulder injury shielding Jun-pyo from an attack, ending her swimming career.
In the climax, Jun-pyo gets selective amnesia after saving Ji-hoo in a car crash, forgetting Jan-di until she falls into a pool, triggering his memories. Ultimately, Jun-pyo's father wakes up, softening Chairwoman Kang. Jun-pyo studies in the US for four years, Jan-di enters medical school, Yi-jung returns from Sweden to reunite with Ga-eul, and Jun-pyo proposes to Jan-di on a beach reunited with F4.