Lee Hak-sung, a math genius and a North Korean defector, works as a guard at a prestigious private school. One day, he meets Han Ji-woo, a student who gets temporarily evicted because of him, and lets him stay with him for a while. Finding out Hak-sung's story, Ji-woo asks to teach math to him, and Hak-sung accepts the request under three conditions. As private math tutoring progresses, Ji-woo learns not only math but the ways of thinking to find the right path to get a result.