Seo-hyun is an ordinary housewife in her late thirties with a ten-year-old son and a successful architect husband. For her, life is a series of banal routines. Her sheltered life is suddenly threatened with the appearance of Woo-in, her much younger sister's attractive new fiancé.

After a high-ranking North Korean official requests asylum, KCIA Foreign Unit chief Park Pyong-ho and Domestic Unit chief Kim Jung-do are tasked with uncovering a North Korean spy, known as Donglim, who is deeply embedded within their agency. When the spy begins leaking top secret intel that could jeopardize national security, the two units are each assigned to investigate each other.