This story follows Jung Hae seong, a highly skilled agent from the National Intelligence Service who once had a reputation as one of the agency’s best operatives. However, after a critical mission goes wrong, his career takes an unexpected turn and he is demoted from his former position. In an attempt to redeem himself, he is assigned to a secret undercover operation with an unusual objective. Hae seong must disguise himself as a student and enroll in a high school while secretly searching for a legendary treasure believed to be the lost gold of King Gojong. As he navigates the challenges of blending into student life, he encounters Oh Soo ah, a teacher at the school who believes that Hae seong closely resembles the boy she once loved in her youth. Their unexpected meeting stirs old emotions and unresolved memories for her, creating a complicated and emotional dynamic. While Hae seong tries to stay focused on his dangerous mission, the past and present begin to intertwine in ways that make his assignment far more complicated than he ever expected.