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After their nobleman father is unfairly accused of being a traitor and killed, Yoon Seo-hwa (Lee Yeon-hee), her younger brother Jung-yoon (Lee David) and their maid, Dam (Kim Bo-mi) are sent to a gisaeng house ("gibang").
Refusing to work as a gisaeng, Seo-hwa agrees after witnessing the brutal beating of her brother.
It is soon announced that her first "patron" will be none other than Jo Gwan-woong (Lee Sung-jae), the same man who betrayed and killed her father.
Just before meeting him, Dam swaps clothes with her so she can run away in the middle of the night, while Dam is raped by Gwan-woong, who sends men to find Seo-hwa.
Gu Wol-ryung (Choi Jin-hyuk), a mystical forest protector and gumiho, notices a disturbance in the forest and finds her unconscious.
He falls in love with her and protects her from Gwan-woong's men by revealing his mystical powers.
When Seo-hwa wakes up, she too falls in love with him and marries him after being told that both Dam and Jung-yoon were able to run away and are safe.
In actuality, Jung-yoon had been hanged, whereas Dam had committed suicide. Wol-ryung, unable to tell her the truth, had lied to her and did not tell her that he was a gumiho.
Wol-ryung decides to become human in order to be with Seo-hwa.
To do so, he must live 100 days without showing his true form to a human, without taking a life (animal or human), and must help anyone that needs aid.
But if he fails, he'll lose any chance of ever becoming human, and will become a demon for the next thousand years.
Wol-ryung successfully lives most of these days following these rules, but one day, Gwan-woong's men find Seo-hwa alone in the forest.
Wol-ryung rushes to help her, and reveals his true form, massacring the soldiers.
Seo-hwa, horrified, leaves him, who is later killed by the righteous soldier Dam Pyeong-joon (Jo Sung-ha), who had been told that the gumiho was murdering innocents.
Seo-hwa soon discovers that she is pregnant with Wol-ryung's child and gives birth to a son.
Realizing that the baby isn't a monster and truly regretting her betrayal to Wol-ryung, Seo-hwa entrusts the baby in the care of a monk, Sojung (Kim Hee-won).
She then confronts Gwan-woong, but is killed.
The infant is found in a basket by the river, and adopted by nobleman Park Mu-sol (Uhm Hyo-sup).
He grows up as Choi Kang-chi (Lee Seung-gi), ostensibly the son of Lord Park's servant Choi (Kim Dong-kyun), but raised as part of the Park family.
Though Mu-sol's wife Lady Yoon (Kim Hee-jung) never warmed up to him, Lord Park loves Kang-chi like his own son, and he is close to the siblings Tae-seo (Yoo Yeon-seok) and Chung-jo (Lee Yu-bi), whom he loves even though she is betrothed to another.
Kang-chi is notorious in the village as a troublemaker, but he is good-hearted and loyal, and beloved by the servants at the Hundred Year Inn, which the Park family runs.
However, Jo Gwan-woong returns to the village. He believes the wealthy Lord Park has hidden treasure inside the Hundred Year Inn, and in his scheme to take over the inn, Lord Park is killed defending Kang-chi.
Gwan-woong throws Tae-seo and his mother in jail, and Chung-jo is sold to the gibang and convinced by the head gisaeng, Chun Soo-ryun (Jung Hye-young), to survive and by using her beauty and wiles, someday gain power and revenge.
After Kang-chi promises Lady Yoon that he will take care of the Park siblings, she makes a futile attempt to stab Gwan-woong and is killed.
Meanwhile Gwan-woong becomes intrigued by Kang-chi and his seemingly super-human strength.
He uses magic to place Tae-seo under a curse that makes him believe that it was Kang-chi who murdered his father and caused the family misfortune.
Dam Yeo-wool (Suzy) and Gon (Sung Joon) had been dispatched to the village by her father, now-martial arts master Dam Pyeong-joon, to investigate a series of murders they suspect Gwan-woong is responsible for.
Yeo-wool runs into Kang-chi: because of the way she's dressed and her skill in martial arts and archery, he initially mistakes her for a man.
Yeo-wool witnesses the events that befall the Park family, and instinctively helps Kang-chi when he is hunted by Gwan-woong's soldiers.
During a fight, a soldier slices off Kang-chi's beaded bracelet, which was used to contain his powers.
Kang-chi transforms into a half-human-half-beast and easily defeats his enemies.
Sojung 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.
Kang-chi is rescued from a death sentence and taken under the wing of Yi Sun-sin (Yoo Dong-geun), a naval commander.
Yi Sun-sin, Master Dam, and the late Lord Park were part of a secret group protecting the Joseon nation against foreign invasion.
Yi places Kang-chi in the martial arts school run by Master Dam, and there he is trained physically and mentally and learns to control his transformations.
Kang-chi and Yeo-wool gradually fall in love.