Jung Hae Seong is a top agent with the National Intelligence Service (NIS). After a serious incident during an operation, Jung Hae Seong is demoted. He is assigned to an operation to locate King Gojong’s missing gold bars. For this assignment, he hides his identity and goes undercover as a student at the prestigious Byeongmoon High School. There, he meets Ms. O Su A, his homeroom teacher. Su A is a contract Korean history teacher with an upright personality and a deep affection for her students. One day, Hae Seong is transferred to Byeongmoon High School and becomes her homeroom teacher. She begins to see her first love, from her childhood, superimposed onto Jung Hae Seong. Her first love caused the deepest wound to her heart.