Jung Hae-Seong (Seo Kang-Joon) 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-A (Jin Ki-Joo) as his homeroom teacher. Oh Soo-A is a contract Korean history teacher with an upright personality and she holds strong affection for her students. One day, Jung Hae-Seong 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-Seong. Her first love caused the biggest wound to her heart.