Hong Le-on (Kim Yoo-jung) disguises herself as a man and goes by the alias Hong Sam-lang, making a living by writing love letters for others. One day, an accidental mistake caused Hong Le-on to get into a lot of trouble. Not only did she owe a huge debt, but she was also in danger. At the same time, Hong Le-on met a scholar named Lee Yeon (Park Bo-gum). The two people who didn't get along well had many conflicts due to various misunderstandings. By chance, Hong Le-on entered the palace and became a eunuch. However, what she didn't expect was that she met Lee Yeon again in the palace. It turned out that this person's true identity was the noble prince. In order to get a breath for his sister, Lee Yeon decided to keep Hong Le-on by his side and wanted to play a good trick on her. As time went by, Hong Lerong and Li Yan shared weal and woe, and actually developed sincere feelings for each other. Li Yan could no longer ignore the throbbing in his heart, and at this time he did not know that Hong Lerong, who he lived with day and night, was actually a girl.