Demoted NIS agent Jung Hae-seong goes undercover as a high school student at Byeongmun High to locate Emperor Gojongs missing gold bars hidden within the school. Posing as a transfer student, he navigates bullying, school politics, and corruption while clashing with his homeroom teacher Oh Su-ah, who suspects his true identity and sees echoes of her lost first love. As alliances form and secrets unravel, the mission exposes deeper conspiracies tied to his fathers disappearance.