Professor Yun's wife gave birth to another little girl. When the 3-year-old Junxi (Song Seung-heon) and his father went to the hospital to visit his mother and sister, he playfully swapped his sister with the baby in the cradle next to him, and a misunderstanding began. Fourteen years later, Junxi has grown into a handsome boy, and his sister Eunxi (Song Hye-kyo) is also very smart and cute. The superior family background makes the two siblings the envy of the school. Xin Ai (Han Caiying), who is in the same class with Eunxi, has a difficult life because her father died early and her brother is idle. The family only relies on the restaurant supported by her mother alone to survive. Xin Ai has always been very jealous of Eunxi. One day she deliberately retaliated against Eunxi and was discovered by Junxi. Eunxi didn't want to make a big deal and rushed to stop her brother and Xin Ai from arguing, but was hit by a truck. In the hospital, Eunxi, who continued to receive blood transfusions, was found to be not the biological daughter of Professor Yun and his wife. After investigation, it turned out that Xin Ai was Professor Yun's biological daughter. Xin Ai learned of this secret and came to Yun's house to recognize her biological parents. Although her adoptive mother Jinghe was very reluctant, Eun-hee returned to Xin'ai's difficult home in order not to make things difficult for her mother, thus starting another chapter of her life...