National Intelligence Service agent Jung Hae-seong (Seo Kang-joon) is demoted after a mission failure. To redeem himself, he infiltrates Byeongmun High School as a student to locate Emperor Gojong's missing gold. There, he encounters his homeroom teacher, Oh Soo-ah (Jin Ki-joo), who notices a resemblance between Hae-seong and her first love, stirring old emotions.