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

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

Yoon Joon-seo returns to his hometown and looks for Choi Eun-seo. Eun-seo has an unwanted encounter with Han Tae-seok.

Tae-seok is upset with Eun-seo going on a blind date for money and takes her to the beach where Joon-seo comes.

Joon-seo feels sorry for Eun-seo growing up in harsh circumstances. Tae-seok's confession bothers him.

Shin Yoo-mi notices that the feelings between Eun-seo and Joon-seo are more than brother and sister.

Joon-seo and Eun-seo decide to stay as brother and sister, but Tae-seok sees her crying in Joon-seo's arms.

Joon-seo and Eun-seo go on a trip for just two of them, and Yoon Shin-ae exposes their relationship to their parents.

Joon-seo announces to his family that he will marry Eun-seo, but everyone is against his decision.

Joon-seo feels guilty for Yoo-mi's breakdown and changes his mind to stay with her, leaving Eun-seo.

Eun-seo lies to Joon-seo that she sees Tae-seok again. Tae-seok knows about it and pretends to be a couple.

Eun-seo learns she has leukemia and Tae-seok begs her asking for a chance to cure her disease.

Joon-seo finds out Eun-seo has leukemia, and Eun-seo heads to the hospital with Tae-seok for her treatment.

Eun-seo tells Joon-seo to love each other forever, only in their hearts. Soon after she goes into a coma.

Eun-seo regains consciousness when Joon-seo comes, and they decide to spend the rest of their time.

Yoo-mi decides to study abroad, and Tae-seok leaves Joon-seo and Eun-seo together. Eun-seo dies on Joon-seo.