Yoon Joon-suh (Song Seung Heon) and Yoon Eun-suh (Song Hye Gyo) always thought they were siblings. Eun-suh is in fact from a different family because she was switched at birth, but no one knows about this fact until one day, when Eun-suh is in a car accident. Her blood is tested only to discover that her blood type is completely different from her brother's proving that they are not related at all.