During that youthful time, Shanghe (Park Yong-ha) and Rouzhen (Choi Ji-woo) were a couple in the class. They often did activities together, and teachers and classmates all knew about it. However, their sweet campus love changed because of one person. On this day, the handsome boy Junxiang (Bae Yong-joon) transferred to Shanghe and Rouzhen's class. Perhaps this was love at first sight. After Rouzhen and Junxiang met, she stubbornly believed that Junxiang was her true first love, so she threw herself into Junxiang's arms regardless of everything, and the two soon fell in love. Unexpectedly, on Christmas Eve, Junxiang's mother decided to send Junxiang to the United States in order to prevent him from recognizing his biological father. Before leaving, Junxiang was unfortunately involved in a car accident when he rushed to meet Rouzhen, and then his mother spread the news of his death. All this made Rouzhen very sad, but she could do nothing about it. Ten years have passed in a blink of an eye, and the former teenagers have grown up. Jun-sang's mother returned to Korea with her husband whom she met in the United States, along with their son Min-hyung. At a cocktail party, Yoo-jin met Min-hyung and found out that he was actually Jun-sang who claimed to have died that day! However, Min-hyung had no impression of Yoo-jin at all...