AKA Eon-deo-keo-beo Ha-i-seu-kul
Ace NIS agent Jung Hae-seong is demoted after a mission failure. He goes undercover at a high school to locate King Gojong's missing gold, meeting his teacher Oh Soo-ah, who sees him as her lost first love, stirring old heartbreak.
Note: This show is in Korean Language but with hardcoded English Subtitles.