The story begins with toddler, Yoon Joon-suh accidentally causing the switch of his sister and another baby when he drops the name cards on the two babies' cribs in the hospital's baby room. A nurse who comes in puts the name cards back incorrectly. The story then jumps forward to the teenage years of the two main characters: Yoon Eun-suh (Moon Geun-young) and Yoon Joon-suh (Choi Woo-hyuk). They are the children of a professor and believe they are biologically brother and sister until Eun-suh gets hit by a truck and needs a blood transfusion. In the beginning of the drama, Eun-suh is portrayed to be very popular in school whereas her rival, Choi Shin-ae (Lee Ae-jung), is smart but does not get the attention she craves from the teacher and classmates. Later, their situations are reversed when everyone discovers that they have been switched at birth. After the doctor runs blood tests on Mr. and Mrs. Yoon for Eun-suh's transfusion, the Yoons find out that Eun-suh belongs to Mrs. Choi, and that Shin-ae is the Yoon's biological daughter. At first, only the parents and brother know about this, but eventually, Shin-ae discovers the truth after she gets into a fierce argument with her non-biological mother over a teddy bear she received from her biological father. Eun-suh only discovers the truth after Shin-ae begged her biological parents to take her in. In the end, the two daughters return to their original birth parents. Shin-ae moves in with the Yoon family, and Eun-suh moves in with Mrs. Choi, (her biological father is dead) who operates a small restaurant living in abject poverty. Shortly after the switch, the Yoon family moves to the United States and Eun-suh loses touch with them. Ten years later, Joon-suh (Song Seung-heon) returns to South Korea as a successful artist. He goes back to the town he lived in, where he encounters his old friend, Han Tae-seok (Won Bin), who stays at the hotel where Eun-suh (Song Hye-kyo) works as a telephone receptionist and hotel maid. Tae-seok, (who doesn't know about the brother and sister mix-up) falls in love with Eun-suh and manipulates her until she is fired from her job. One day Eun-suh sees Joon-suh and follows him to the beach where he is with Yumi (his fiancée) and Tae-seok, and the two "siblings" finally meet again after ten years. Eun-suh and Joon-suh pretend to have a sibling relationship in front of the others, but they meet each other secretly and fall deeply in love, though she always refers to him as "older brother." Shin-ae (Han Chae-young) finds out about their relationship and exposes the two after she finds a love letter Eun-suh wrote to Joon-suh. The two decide to stay together, but are soon forced apart again because their parents are against the union, and Yumi hurts herself and blackmails Joon-suh with suicide to hold on to him. As a fight between Joon-suh and Tae-seok erupts over their love for Eun-suh, she discovers she has leukemia (the same fatal condition that killed her biological father). She doesn't tell anyone except Tae-seok, who offers to pay for her treatment. When her health deteriorates, the others begin finding out the extent of her condition. Eun-suh soon falls into a coma. Joon-suh finds out about Eun-suh's health and reacts with shock and fear. Tae-seok forces Joon-suh to try to wake up Eun-suh. Luckily, she does wake up, but is too weak to follow the treatment. When it is clear that there is no hope, Joon-suh takes her home so she can spend her last days with him. At this point, Yumi finally lets go of Joon-suh. Joon-suh proposes to Eun-suh and they get married. Eun-suh dies as Joon-suh carries her around the beach where they spent her birthday as teenagers. Before Eun-suh dies, she tells Joon-suh to move on and continue living. However, Joon-suh, dazed and grief-stricken by the death of his love, is struck by a truck in the same place as Eun-suh's accident during her teenage years. Whether he lives or dies is unknown.

• published: 13 Apr 2016
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