Although An Dae-seong has an excellent memory and attention span, he has been unable to pass the civil service exam for several years. With the support of his mother and his police girlfriend, he continues to try, and he currently works part-time at his mother's store on the outskirts of Seoul. When a murder occurs in the neighborhood, Dae-seong realizes that the only clue that can help identify the perpetrator is a receipt from their store.