Da Ran (Lee Min Jung) is a girl who is engaged to the brilliant Dr. Seo Yoon Jae (Gong Yoo). She is going to start her work as a temporary teacher in a school and by accident meet a transfer student from the USA named Kang Kyung Joon (Shin Won Ho) and because of some coincidences they get to know each other. Recently Yoon Jae behaved coldly to Da Ran and Kyung Joon finds out how much she suffers so he try to help her improving her relationship with Yoon Jae, but he and Yoon Jae are involved in a car accident and both fall into the river. Yoon Jae tries to save Kyung Joon. In the hospital, Kyung Joon wakes and find himself in the body of Yoon Jae, as his body is lying in hospital and everybody thinks he is in a coma. The only one whom he can tell that he is not Yoon Jae is Da Ran.
How he should act as Yoon Jae and not young Kyung Joon and how Da Ran should behave him from now on?