Cha Dae-Woong (Lee Seung Gi) accidentally releases a gumiho, a legendary fox with nine tails who was trapped inside of a painting of Grandma Samshin (三神). He does this by adding nine tails to the fox. The gumiho wanted to be human before, but was not able to fulfill the requirement of finding a husband, because a rumor was spread claiming that she eats the livers of humans. During their first encounter, Dae-woong experiences a life-threatening fall, and the gumiho saves his life by giving him her fox marble (여우 구슬). Later on, Dae-Woong wakes up and meets a pretty girl, not knowing that she is the gumiho that he released. When Dae-woong finds out about what he did, he must try to keep her happy and hide the fact that she is a gumiho from everyone. However, because Dae Woong has gumiho's bead that saved his life, he has no choice but to let her stay with him.
As the story goes on, the legendary gumiho, later called Miho, wants to be human. Miho (Shin Min Ah) is informed by a veterinarian named Park Dong Joo (who is half human, half monster) that in order to fully transform into a human, she must drink human blood and give a human her fox marble so that he keeps the fox-marble in his body for a full 100 days. Moreover, Dae Woong will not be able to date anyone else but gumiho within the 100 days. However, Dong Joo does not tell Miho that, after the 100 days pass, in order for her to become human, Dae-woong must die. Trouble brews when Miho and Dae-woong eventually fall in love.