[1][2][3][4] The fusion martial arts action historical drama is about a half-man half-monster who is searching for a centuries-old book that according to gumiho legend, contains the secret to becoming human.

Gu Wol-ryung (Choi Jin-hyuk), a mystical forest protector and gumiho, finds Seo-hwa unconscious and, as he had fallen in love with her, protects her.

Gwan-woong throws Tae-seo and his mother in jail, and Chung-jo is sold as a gisaeng and convinced to use her beauty and wile to someday gain power and revenge.

So-jung later tells Kang-chi the truth about his origins, including the mystical and elusive Gu Family Book, which holds the secret to becoming fully human.

Yi Sun-sin, Master Dam, and the late Lord Park were part of a secret group protecting the Joseon nation against foreign invasions.

Yi places Kang-chi in the martial arts school run by Master Dam, there he is trained physically and mentally and learns to control his transformations.

But several obstacles remain in their way: the continuing villainy of Gwan-woong; the knowledge that it was Yeo-wool's father who killed Kang-chi's father; the reappearance of Wol-ryung who has turned into a soul-sucking demon with no memories who can only be killed by his son; and Sojung's warning that Yeo-wool is fated to die if she stays by the side of the man she meets beneath the blossoming peach tree under a crescent moon, which is none other than Kang-chi himself.