Undercover High School

Hae Seong's team faces danger after a failed operation. To save his NIS career, he accepts a near-impossible mission: infiltrate Byeongmun High, an elite school, and recover ten tons of gold worth $80 million. "Operation Undercover High School" begins.

At Byeongmun High, Hae Seong nearly blows his cover searching for gold in the creepy old building, narrowly avoiding his sharp homeroom teacher, Su A. Rumors and tight security convince him the gold is there. While handling bullies, Hae Seong plots his next move to uncover the building's dark secrets.

Hae Seong 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 Seong under close watch.

Hae Seong 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 Seong 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.

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 Seung is caught off guard when he recognizes 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 Seung 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 Seung's strong opposition to her involvement.

After uncovering the possibility of a mole within the NIS, Hae Seung 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 Seung uncovers the connection between the gold bars and Chairwoman Seo, prompting him to vow and end her sinister plans

As Su A recovers in the hospital following the attack during the break-in, Hae Seong and the team uncover the culprit, Lee Jun Ho, the mathematics teacher. When Hae Seong detects his father's notebook is missing, he realizes that Seo Myeong Ju was the one who orchestrated the break-in. After Lee Jun Ho's arrest and confession, the revelation of the school's grade-rigging scandal throws the institution into chaos. Amid the turmoil, Hae Seong stumbles upon a crucial clue to the final scary story.

To everyone's shock, Seo Myeong Ju returns to the school. Unimpressed by Hae Seong, she expels him and demonstrates her power by dismantling his team. However, refusing to back down, Hae Seong and the team, including Su A and her mother, devise a daring plan to infiltrate the school once more to retrieve the hidden gold bars.

The Devastating Truth The gold bars vanish just as they are within reach, and Seo Myeong Ju suspects Hae Seong has them. In the vault, Hae Seong makes a heartbreaking discovery of his father's remains. Overcome with grief, he finally finds closure, but a shocking truth emerges. He learns Director Kim Hyung Bae was behind it all. Determined to uncover the whole story, Hae Seong heads to confront him alone

Where the Stars Lead Despite getting badly injured, Hae Seong manages to call Su A and his team. They conclude that the only evidence they can secure is the flash drive that contains the voice recording of Principal Park murdering Director Kim Hyung Bae. After finding out that an arrest warrant had been issued for her, Seo Myeong Ju decides to take a special measure to end everything.