The eldest princess Li Huaiyu was dedicated to assisting the young emperor to protect the country. But she was wrongly accused of plotting to murder a high ranking court official and therefore sentenced to death by the emperor.
She reincarnated as the fourth daughter of the Bai family. She tried to seek revenge by getting close to Jian Xuanjin who she thought was the person who gave her the poisonous wine, that killed her.
Therefore they joined forces to search for the truth and got to know each other better. As a result, they developed a love hate relationship.