Hong Ra On, disguises herself as a man and counsels men on dating. Due to a love letter she wrote for a client, she meets Lee Yung---future Crown Prince Hyomyung. Hong Ra On is unaware that he is the Crown Prince and Lee Yung is unaware that Hong Ra On is a woman. The Crown Prince becomes interested in Hong Ra On. As nobody is aware of Ra On being a woman, she is sold to the palace and becomes an eunuch...
Raon disguises herself as a man named Samnom and does all kinds of work to earn money, pay off the debts and find her mother. One day, she is kidnapped by the moneylenders, and they sell her to the Office of Eunuch Attendants to get their money back.
To become the official eunuch, the trainees have to pass three exams which are the physical fitness, the written exam and the character test. They will be kicked out of the palace once they fail at least one of them.
The King personally orders Raon to deliver his response of Consort Park's letter, and it turns out to be a secret letter written with apple vinegar.
Yeong suggests the King and the Ministers to get the Qing Dynasty's approval of his taking the throne in the interim after the King's birthday festival.
The mysterious maiden who performed the solo dance during the King's birthday festival is nowhere to be found, and the Crown Prince suspects Eunuch Hong to be the one.
Eunuch Ma secretly tells Envoy Mok Taegam that Eunuch Hong is a girl and the dancer he has been desperately looking for.
Eunuch Ma and Wolhee must end their relationship because a eunuch and a court lady's love cannot be done.
Yeong is about to start ruling as an interim king, but the conflict regarding the national exam between Yeong and Kim Heon deepens.
Yeong continues to postpone the national exam to stop the corruption, which leads to more deepened conflicts with Kim Heon.
Raon tells Yeong a story about the mermaid who falls in love with a prince and turns into sea-foam in the end.
The King persuades Yeong that he must marry Minister Cho's daughter to have more people on his side.
Premier Kim's servant discovers that Hong Gyeongnae's daughter is called Hong Raon and she is currently in the palace.
Raon finds out that she is Hong Gyeongnae's daughter and realizes she can never be safe in the palace.
The Crown Prince finds out that Eunuch Hong is actually Hong Gyeongnae's daughter, but he cannot believe she approached him on purpose as a spy.
Yeong is heartbroken that he has become the person who only makes life difficult for Raon.
The palace guards capture Hong Gyeongnae alive and prepare for the interrogation.
With Chief Eunuch Han and Byeongyeon's sacrifice, Raon and her father are able to escape from the palace successfully.
Premier Kim tries to poison Yeong and make Yunseong a new crown prince, but Yeong recovers from his illness thanks to Master Jung.