A heart's first awakening.
Two men and two women, their relationship begins in tragic fates, and it grows into love, agony, and parting.
Professor Yoon's couple learns their child has been switched through blood tests after a car accident.
Professor Yoon and Soon Im switch their children, but they only suffer from unfamiliar circumstances.
Jun Seo returns to his hometown and looks for Eun Seo. Eun Seo has an unwanted encounter with Tae Seok.
Tae Seok is upset with Eun Seo going on a blind date for money and takes her to the beach where Jun Seo comes.
Jun Seo feels sorry for Eun Seo growing up in harsh circumstances. Tae Seok's confession bothers him.
Yu Mi notices that the feelings between Eun Seo and Jun Seo are more than brother and sister.
Jun Seo and Eun Seo decide to stay as brother and sister, but Tae Seok sees her crying in Jun Seo's arms.
Jun Seo and Eun Seo go on a trip for just two of them, and Sin Ae exposes their relationship to their parents.
Jun Seo announces to his family that he will marry Eun Seo, but everyone is against his decision.
Jun Seo feels guilty for Yu Mi's breakdown and changes his mind to stay with her, leaving Eun Seo.
Eun Seo lies to Jun 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.
Jun Seo finds out Eun Seo has leukemia, and Eun Seo heads to the hospital with Tae Seok for her treatment.
Eun Seo tells Jun Seo to love each other forever, only in their hearts. Soon after she goes into a coma.
Eun Seo regains consciousness when Jun Seo comes, and they decide to spend the rest of their time.
Yu Mi decides to study abroad, and Tae Seok leaves Jun Seo and Eun Seo together. Eun Seo dies on Jun Seo.
