18 year old teen teens this is a 30 year old adult male at any moment and what you get is a drama romance fantasy story.
Gil Da-ran (Lee Min-jung) is a struggling substitute teacher trying to get her certificate.
She is engaged to a kind and handsome doctor, Seo Yoon-jae (Gong Yoo); it is a fairy-tale-come-true until one month before the wedding, Yoon-jae inexplicably becomes distant. At the same time, Da-ran meets an 18-year-old transfer student from the United States, Kang Kyung-joon (Shin Won-ho), who's got an attitude towards everyone except Da-ran.
Kyung-joon gets into a fateful car accident with Yoon-jae and the two of them drive over the guard rail and into the deep waters.
Yoon-jae tries to save Kyung-joon but in the process, Yoon-jae ostensibly dies and Kyung-joon ends up in a coma.
Kyung-joon soon wakes up finding himself in Yoon-jae's body.