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.