Jung Hae Seong is one of the most successful field agents of the National Intelligence Service (NIS). However, after a major incident during an operation, he is demoted. As a result, he is assigned to find the location of King Gojong's lost gold bars. For this mission, he conceals his identity and takes on a secret role as a student at the prestigious Byeongmoon High School. There, he meets O Su A, a contract history teacher with a strong personality who has a deep affection for her students. One day, Hae Seong transfers to Byeongmoon High School, and Su A becomes his homeroom teacher. Over time, Su A begins to see her first love from childhood in Jung Hae Seong. However, that first love has left the deepest wound in Su A's heart.
