In the reign of the thirteenth emperor of the glittering country, a lonely girl named Xiao Hu fights desperately to protect her younger sister, Xiao Hua. Her innate combat talent catches the eye of Cang Ruiyu, a high-ranking official of the Black Guard Bureau. He takes in both sisters and trains Xiao Hu into a formidable warrior, eventually assigning her to serve as the emperor’s personal guard, clad in black. The emperor, Zhu Xiao, proves to be a flamboyant and high-handed ruler, but Xiao Hu does not flinch before him, and he grows intrigued by her fearless demeanor.

A series of unsettling events begins to shake the capital: the mysterious deaths of eunuchs and the secret circulation of a strange drug known as the Tongxian Pill. Cang suspects the remote frontier region of Hongsha Terrace is behind the plot. To investigate, he sends Xiao Hu to infiltrate the imperial harem under the false identity of Fufu Caiten, a low-ranking consort. Her mission is to get close to Bao Caiten, a consort from Hongsha Terrace, and uncover the conspiracy.

As Xiao Hu navigates the treacherous environment of the harem, she must balance her duty to the Black Guard, her hidden feelings for the man who raised her, and the emperor’s deepening affection for her. The story follows the intersecting paths of Xiao Hu, the dedicated official Cang, and the growing emperor Zhu Xiao as they confront palace intrigue, a web of dangerous drugs, and the rivalries of the harem, all while each of them wrestles with their own converging desires.