Jung Hae Seong is an ace field agent for the National Intelligence Service (NIS). After a big incident occurred during an operation, Jung Hae Seong receives a demotion. He is placed into an operation to locate the whereabouts of 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 Teacher O Su A as his homeroom teacher.

Su A is a contract Korean history teacher with an upright personality and she holds strong affection for her students. One day, Hae Sung is transferred to Byeongmoon High School and she becomes his homeroom teacher. She begins to see her first love, from her childhood days, overlap with Jung Hae Seong. Her first love caused the biggest wound to her heart.