Mei Zhuyu, a calm and reserved nobleman from a lesser branch of the Mei family, arrives in Chang'an as an official at the Bureau of Mystic Investigations. During a banquet, he meets Wu Zhen, the bold and unruly younger sister of the Empress and secret Guardian of the Demon Underworld. Both hide their true identities. Wu, a half-demon striving to keep peace, and Mei, a skilled demon slayer, seek the truth behind the fire 18 years ago. As bone demons rise and unrest grows, they fight side by side, forming a bond that deepens into love until the Changxi Palace launches a purge, forcing them to face each other's secrets and the past they share.

Set during the Ming dynasty during the rule of Yongle Emperor, it tells the story of a young girl named Yao Zijin, who entered the palace to become a maid of the food bureau. In the deep palace, she remains determined and slowly works her way up while perfecting her craft; while making friends and encountering an unexpected romance at the same time.

Cast members work together to escape a secret room.