Gil Da-ran is a substitute teacher who is engaged to the perfect doctor, Seo Yoon-jae. However, their lives are turned upside down when Yoon-jae is involved in a car accident with Kang Kyung-joon, an 18-year-old transfer student from the United States. Following the accident, Kyung-joon's soul ends up inhabiting Yoon-jae's body, leaving the teenager trapped in the body of his teacher's fiancé. As Da-ran tries to navigate this bizarre situation, she finds herself caught between her feelings for her original fiancé and the growing bond she develops with the teenage soul trapped inside him. The series explores themes of identity, maturity, and the complications of love as they search for a way to return Kyung-joon to his own body while uncovering the secrets behind the accident.