Legend has it that in ancient times, all the mountains and rivers were guarded by guardian spirits. The guardian of Jirisan is Gu Yueling (Choi Jin Hyuk), who witnessed an incident in the human world and fell in love with the heroine Yoon Seo Hwa (Lee Yeon Hee), even at the cost of giving up his eternal life. In order to become a human, Gu Yueling must abide by the three rules of the Nine Family Book, but because of an accident, Gu Yueling failed to comply, leaving Seo Hwa and their newborn child behind. Seo Hwa abandoned her son in the river and was adopted by the wealthy businessman Park Moo-ryul, who grew up to be Choi Kang-chi (Lee Seung-gi), who is passionate, impulsive and curious. On the other hand, Tan Ruwei (Bae Suzy), the instructor of the Invisible Sword Hall, who is dissatisfied with the definition of women in the era and wants to live out his own personality, met Kang-chi because of an investigation and developed a feeling of pity for him, and the two also developed love in a series of events. The half-human, half-beast Kang-chi is looking for a way to become a human in a series of events, and he also gains courage, love and strength in the process.