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.

Hae-sung attends the anti-bullying committee session with a key witness who clears his name, while Dong-min finally finds the courage to stand up for himself. With his name vindicated, Hae-sung uncovers the existence of another grandfather clock hidden in the student council room, and the only way to get closer to it is by joining the student council. With the help of his team, he tries to meet the strict requirements.

Hae Seong and the team grow increasingly concerned that Su A's suspicions could threaten his undercover mission. However, Hae Seong defies the odds and successfully gains a spot in the student council. With this new role, his focus shifts to unraveling the mystery behind the old clocks. Meanwhile, a shadowy figure emerges, threatening to expose Hae Seong's true identity

The school's security guard dies, leaving Hae Seong a notebook belonging to his missing father. The revelation is shocking, and he storms the NIS for answers. The answer pushes him away from the mission, and he takes time off. However, he resolves to continue but returns only to discover Su A witnessed it all

Hae Seong is caught off guard when he recognises Su A's childhood name, Bong Ja, which stirs memories of his first love and distracts him from his focus. Despite the emotional turmoil, Hae Seong pushes forward with the mission to uncover the secrets behind the third ghost story. However, a missing key component forces Su A to join the mission despite Hae Seong's strong opposition to her involvement

After uncovering the possibility of a mole within the NIS, Hae Seong becomes increasingly distrustful of those around him, causing him to question the mission's integrity. Meanwhile, Su A exposes a secretive scheme within the school that guarantees top grades for VIP students. As the web of secrets deepens, Hae Seong uncovers the connection between the gold bars and Chairwoman Seo, prompting him to vow to end her sinister plans