A female college student named Lee Dam accidentally swallows the marble of a male gumiho (a mythological nine-tailed fox) named Shin Woo Yeo, who was close to achieving his goal of becoming human. If a human swallows the marble, they can only live for one year, and the marble will break, preventing the gumiho from becoming human. Thus, Shin Woo Yeo suggests living together to Lee Dam, beginning their unexpected romance. Shin Woo Yeo is a gumiho with good looks, sturdy physique, good manners, and a sweet charm who has lived for 999 years.