NIS agent Jung Hae-seong, undercover as a student, narrowly avoids exposure by his astute homeroom teacher, Oh Soo-ah. While investigating a heavily secured, eerie building on campus for Emperor Gojong's missing gold, Hae-seong confronts school bullies, intensifying his mission's complexity. Meanwhile, Soo-ah becomes increasingly suspicious of Hae-seong, sensing a resemblance to her first love, adding emotional depth to their interactions.