Tae Ho and Sang Goo are two men who constantly fought at work and hates each other even more now that they're both the heads of rival companies. When their children Do Hee and Young Joon get married, the bickering fathers are forced to become in-laws.

Secret Investigation Record is based on cases detailed in the Annals of Joseon History with a bizarre twist; these incidents incorporate mysterious, supernatural elements that cannot be explained through science. Kim Hyung Do is an inspector of the state whose single-minded focus on his investigations into administrative misdeeds bring him into direct contact with these strange mysteries. Heo Yoon Yi is a government employee who aids him and attempts to understand these scientific impossibilities with an open mind, in contrast to his cynicism.