Cha Dae-woong (Lee Seung-gi) is a normal college student, with ambitions of becoming an action star. Until one day, he accidentally releases a gumiho (Shin Min-ah), a legendary fox with nine tails who was sealed inside a painting by Samshin Grandmother (三神). Fleeing the scene, Dae-woong suffers a life-threatening fall and is only saved by the gumiho giving him her “fox bead” (여우 구슬). When Dae-woong awakes and meets a mysterious and very beautiful girl, he initially treats her as eccentric or mentally ill, before finally understanding that she is a gumiho in human form. According to myth, gumihos eat men’s livers, so Dae-woong is scared out of his wits, and the gumiho takes advantage of his fear by sticking by his side. He names her “Mi-ho,” and keeps her happy by buying her beef and hiding her true identity. Mi-ho wants nothing more than to become human, so they strike a deal: she’ll lend him her mystical fox bead so that with his increased speed and strength, he’ll be able to do difficult stunts, and in exchange he’ll help her become human. As time passes, they get to know each other better and Dae-woong is charmed by how Mi-ho looks at the world with childlike wonder.
Enter Park Dong-joo (No Min-woo), a half-human, half-supernatural being, whose cover identity is as a working veterinarian. Dong-joo tells Mi-ho that she’ll be able to turn human if she follows these instructions: She must drink Dong-joo’s blood then place her fox bead inside a human’s body for 100 days. Once she does this, her supernatural self will start to die, gradually. Her gumiho energy will slowly empty away while her bead will live in the human for 100 days and take in his energy. And when she reclaims her bead, she will become human. Mi-ho happily complies, and Dae-woong accepts her bead.
However, Dong-joo deliberately withheld the information that this process will result in the human’s death. Mi-ho and Dae-woong thus fall in love without knowing the consequences of fulfilling her goal: in 100 days, Dae-woong will die if Mi-ho becomes fully human, but Mi-ho will die if the bead isn’t returned