Kim Seok-go showed a lot of promise as a brilliant mathematician when he was in school, resolutely focused on his studies rather than on friends throughout his childhood. Now in his 30s, he's an ordinary high school math teacher, a far cry from the promising future of his youth. Seok-go is solemn and introverted, and his morning exchanges with Baek Hwa-sun, the cafe employee he buys lunch from, is the brightest part of his day. When Hwa-sun's ex-husband mercilessly beats Hwa-sun and her niece, Hwa-sun kills him. Seok-go overhears the fight from his house next door and decides to cover up the killing and protect her from the police. He uses his genius in meticulously planning the perfect alibi for her, and thanks to his efforts, Hwa-sun is cleared in the case. However, the detective in charge, Jo Min-beom, believes that Hwa-sun is guilty and follows his intuition despite the lack of evidence. Min-beom also happens to have gone to the same high school as Seok-go, and when he finds out that his old school friend lives next door to the prime suspect, he starts digging deeper into Seok-go's life.