Su-yeon, a cellist in an orchestra led by her fiancé and conductor Sung-jin, disappears one day, leaving behind only a video recording. Sung-jin is devasted over the loss of Su-yeon, but feels a strong attraction to Mi-ju, a cellist who fills in for his fiancée. Then one rainy night, Sung-jin and Mi-ju get swept away by their mutual desires for each other and commit an unforgivable act at Su-yeon's house.

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.