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Set in an alternate Joseon dynasty, Jo Yang-sun (Lee Yu-bi)

is the daughter of a nobleman whose family loses everything when her father is framed for treason. To make ends meet, Yang-sun begins cross-dressing as a male bookseller and meets the handsome and mysterious scholar Kim Sung-yeol (Lee Joon-gi),

who works at the Hongmungwan. Kim Sung-Yeol has spent 120 years as a Guardian Vampire who is responsible to get rid of any vampire who disobeys the rules. As a guardian vampire, he is able to go out in sunlight as long as he wears his special black robe. Sung-Yeol continues to be haunted by the long-ago death of his first love, Lee Myung-hee (Kim So-eun),

who sacrificed herself for him 120 years ago. Sung-Yeol decides to pay Yang-Sun a high price if she is able to find the memorandum of his best friend, the late crown prince Jonghyun, which contains the secret to killing Gwi (Lee Soo-hyuk)

in order to avenge the death of his family and lover. After the second meeting with Yang-sun, Sung-yeol find out that Yang-sun is a girl. However he decides to keep it as a secret to avoid awkwardness with Yang-sun. Yang-sun tries very hard to find the memorandum as she promised as she also needs money to cure her sister's leg.

On a trip to find the old book seller from Yuan with Yang-sun, Sung-yeol meets Choi Hye-ryung, Myung-hee's present-day doppelgänger who is an aloof nobleman's daughter. Hye-Ryung's father is a right minister of the king. He has the highest authority in the king's court compared to other ministers. He has also been loyally serving Gwi for a long time. In order to prove his loyalty, he even sacrificed Hye-ryung when she was 10 years old to become Gwi's food toy. However, due to her loyalty and gentle treatment to Gwi, he has unconciously fallen in love with Hye-rung and secretly plans to change her into a vampire, even though he already has 100% control of the royal court. Gwi resides in the underground royal palace and uses his powers and political machinations to prevent the Crown Prince (Shim Chang-min)

from ascending to the throne, as he knows that the crown prince Lee Yoon secretly plans to get rid of him like his late ancestor, Crown Prince Jonghyun.

Gwi is also aware of Sung-yeol's plan to kill him. He becomes furious when he learns that Sung-yeol has gotten close to finding the secret to killing him. Gwi tries to harm Yang-sun several times, but Yang-sun is saved by Sung-yeol, since Sung-yeol also has the ability to sense Gwi's presence. Gwi becomes anxious after he smells Yang-sun's blood and decides to get rid of Yang-sun as a precaution. After a while, Sung-yeol secretly falls in love with Yang-sun after he discovers that Hye-Ryung is one of Gwi's followers.

Jo Seung, Yang-sun's adopted father, is the secret keeper of the Crown Prince JongHyun's memorandum. Jo Seung is a former servant of Yang-sun's biological father who is the right minister who served the late crown prince (Lee-Yoon's father).

He was changed into vampire by Gwi as a punishment and tries to harm Yang-sun. Unknown to Sung-yeol, Yang-sun was saved by him 10 years ago and Sung-Yeol killed Yang-sun's father to save her. Although she has been bitten by her father, Yang-sun/Seo-jin did not change into a vampire, as she has the blood of Gwi. Yang-sun's biological mother is the direct descendant of Gwi from her mother's relations. Her biological mother had raised her as a boy in order to protect her, because she knows that her daughter is needed to be sacrificed as a weapon to kill Gwi.

What is the secret plan in the Crown Prince Jonghyun's memorandum? How can Sung-yeol save Yang-sun from being sacrificed? Will Lee Yoon become a puppet king controlled by Gwi like his ancestor? Will Sang-yeol and Yang-sun lived happily ever after?