The plot is quite full of substance, especially in the beginning, when En Xi and Xin Ai, two very different girls in the same class at high school, experience turbulent changes. Jun Xi and En Xi are two happy siblings, having grown up in a well-off Korean family. Xin Ai, however, has had an unhappy childhood. Her poor and strict single mother owns a run-down restaurant, while her good-for-nothing brother beats her up. Destiny has plans for En Xi and Xin Ai, however. The two girls share one thing in common: the same birthday. En Xi's parents soon find out, after blood samples, that En Xi and Xin Ai were switched at birth and Xin Ai is their true daughter. At first, their parents try to ignore the truth, but soon discover their devotion to their biological daughter, Xin Ai. They bring her home, but Jun Xi does not accept his new sister, having had a great relationship with his sister En Xi. Feeling rejected, En Xi decides to leave and join her biological mother. To run away from their problems, the Yin family decides to travel to America, leaving En Xi alone in Korea.
Years later, after En Xi (now played by Song Hye Kyo) is around 20, the Yin family returns from the States. Jun Xi (now played by Song Seung Hun) is currently an artist, engaged to his girlfriend Shen You Mei (Han Na Na). Meanwhile, En Xi works in a hotel. There, she becomes acquainted with Han Tai Xi (Won Bin), a wealthy, good-looking bachelor with an interest in En Xi. It so happens that Tai Xi is best friends with Jun Xi, who is back in Korea searching for his long lost "sister". Through Tai Xi, the two end up meeting, and soon discover their relationship is deeper than siblings. Their love, however, is not well-taken. Jun Xi's engagement to You Mei suffers, and Tai Xi, who has an interest in En Xi, feels heartbroken. Xin Ai (now played by Han Chae Young), has also returned to Korea with her MBA degree, but soon discovers En Xi has yet again stolen her spotlight. Additionally, Xin Ai has always liked Tai Xi, who is deeply in love with En Xi. The Yin parents are thrilled to see En Xi again, but disapprove of the relationship between En Xi and Jun Xi. The two elope, but soon return because of their responsibilities to their friends and family.
