The fox spirit doctor Xin Yi was born without tears and abandoned. Xie Heng, the Imperial Medical Academy's chief, is poisoned by the deadly Thousand Mechanisms Lure and needs a medicinal fox as an ingredient. He rescues Xin Yi, claiming he can cure her inability to cry, but in truth he intends to use her as a living ingredient.
After arriving in the capital, Xin Yi uses her gifted palate to detect that the poison inside Xie Heng is connected to the State Preceptor's residence. She also discovers that he is the boy who gave her candy on a snowy night ten years ago. Xie Heng plans to discard her once he has finished using her, but each time his poison flares up, she feeds him blood from the tip of her tongue, and he gradually loses control and falls for her.
At the climax, Xie Heng storms the State Preceptor's residence alone to protect Xin Yi, only to discover that the poison-maker is Xin Yi's own elder sister, driven mad by torture. In their desperate moment, Xin Yi weeps in her anguish to save Xie Heng. Her tear ducts burst, and her tears become a life-giving cure that dispels the poison. In the end, Xie Heng resigns from office and helps Xin Yi establish the Demon Doctors' Bureau. Together, they defy the divide between humans and demons and spend their days side by side in the mortal world.
