Ji Woo, an outcast in a prestigious private high school, does not fit in due to different social backgrounds than his well-to-do classmates. One day, he meets Hak Sung, the school’s security guard who is actually a mathematical genius defected from North Korea, now living with his past shut away. Ji Woo asks Hak Sung to teach him math, and although reluctant at first, Hak Sung eventually agrees to it. Finding each other in their most difficult times, Ji Woo and Hak Sung open up to one another during their special math lessons, but their friendship is at risk when Ji Woo is framed for an incident in school, and Hak Sung is faced with people prying on his past.