Jung Hae Sung 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 prestigious Byeongmoon High School. There, he meets Teacher Oh Soo Ah as his homeroom teacher. Soo Ah 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 in Jung Hae Sung. Her first love caused the biggest wound to her heart.