Professor Yoon's couple learns their child has been switched through blood tests after a car accident.

Professor Yoon and Kim Soonim switch their children, but they only suffer from unfamiliar circumstances.

Yoon Joonseo returns to his hometown and looks for Choi Eunseo. Eunseo has an unwanted encounter with Han Taeseok.

Taeseok is upset with Eunseo going on a blind date for money and takes her to the beach where Joonseo comes.

Joonseo feels sorry for Eunseo growing up in harsh circumstances. Taeseok's confession bothers him.

Shin Yumi notices that the feelings between Eunseo and Joonseo are more than brother and sister.

Joonseo and Eunseo decide to stay as brother and sister, but Taeseok sees her crying in Joonseo's arms.

Joonseo and Eunseo go on a trip for just two of them, and Yoon Shinae exposes their relationship to their parents.

Joonseo announces to his family that he will marry Eunseo, but everyone is against his decision.

Joonseo feels guilty for Yumi's breakdown and changes his mind to stay with her, leaving Eunseo.

Eunseo lies to Joonseo that she sees Taeseok again. Taeseok knows about it and pretends to be a couple.

Eunseo learns she has leukemia and Taeseok begs her asking for a chance to cure her disease.

Joonseo finds out Eunseo has leukemia, and Eunseo heads to the hospital with Taeseok for her treatment.

Eunseo tells Joonseo to love each other forever, only in their hearts. Soon after she goes into a coma.

Eunseo regains consciousness when Joonseo comes, and they decide to spend the rest of their time.

Yumi decides to study abroad, and Taeseok leaves Joonseo and Eunseo together. Eunseo dies on Joonseo.