Caiyu (Ha Ji-won) is the daughter of a criminal. She was separated from her brother during the escape and was sent to Huangfu Yun's house (Lee Seo-jin) to become his personal maid. Fifteen years have passed in a flash. Huangfu Yun and Caiyu have grown up. Over the years, whether Huangfu Yun went up the mountain to practice martial arts or entered the thief arrest hall as an official, Caiyu has never left his side. The friendship between the two is as high as the mountain and as deep as the sea. However, the huge difference in identity and status prevents them from ever coming together. Caiyu chose to silently support Huangfu Yun behind the scenes. The two joined forces to solve strange cases one after another, and the handsome and intelligent Huangfu Yun also won the admiration of Lanxi (Cha Yingren), the daughter of his boss General Zhao. Caiyu is investigating and hunting down a man named Zhang Chengbo (Kim Minjun). Zhang Chengbo actually became interested in Caiyu, who is as brave as a man, but he didn't know that Caiyu's true identity is actually the sister he has been looking for.