After a botched operation puts Hae-sung and his team in serious jeopardy, the only way for him to salvage his career at the NIS is to accept an assignment that seems almost hard to believe. Hae-sung and his team must infiltrate Byeongmun High School, one of Korea's most elite private institutions, to recover ten tons of gold bars worth a staggering 80 million dollars. With everything on the line, Hae-sung puts on a school uniform, and "Operation Undercover High School" begins.

Hae-sung comes dangerously close to blowing his cover while searching for the hidden gold in Byeongmun High School's eerie old building, barely escaping detection by Soo-ah, his sharp-eyed homeroom teacher. As he hears chilling rumors and faces tight security surrounding the building, he believes that's where he'll find the gold. While dealing with school bullies, Hae-sung begins plotting his next move to uncover the dark secrets lurking inside the old building.

Hae-sung uncovers a crucial clue hidden within the old building, but Chairwoman Seo quickly learns its potential connection to the elusive gold bars. Sensing the connection, she keeps Hae-sung under close watch. Meanwhile, as Hae-sung delves deeper into the ghost stories surrounding the school, he finds himself caught in the escalating tension between Tae-soo and Dong-min, whose conflict ignites chaos throughout the school.