The medicine fox Xin Yi was born without tears and was abandoned. The Imperial Hospital diagnosed Xie Heng with the highly poisonous "Qianji Yin" and needed to use the medicine fox as a medicine fox. Xie Heng rescued Xin Yi and lied that he could cure her acronymia, but in fact he lured her into living medicine.
After entering the capital, Xin Yi used his spiritual tongue to taste that the toxins in Xie Heng's body were related to the Imperial Preceptor's Office, and even discovered that he was actually the boy who gave him candy on a snowy night ten years ago. Xie Heng originally wanted to use it and then abandon it, but she lost control and sank as she fed him the blood from the tip of her tongue again and again.
At the climax, Xie Heng breaks into the Imperial Preceptor's Mansion alone to protect Xin Yi, and discovers that the poisoner is actually Xin Yi's own sister who was tortured and went crazy. In desperate situation, Xin Yi cried in grief to save Xie Heng, and his tear orifices burst and turned into tears to detoxify. In the end, Xie Heng resigned and helped Xin Yi establish the "Demon Medical Office". The two broke through the different paths of humans and demons and stayed together in the world of mortals.
