In ancient times, there was a war between gods and demons. Both tribes suffered heavy losses and had to retreat to their respective territories. Before the demon tribe returned to the demon world, the sister of the demon king and the eldest princess of the demon tribe, You Ji, used her soul, menstrual blood and cultivation to merge into the fine iron of the demon world and forge the demon sword Xueyou. From then on, , time began to spread a legend. The legend said, "Whoever wins the Blood Netherworld will win the world. If the Blood Netherworld appears, the demon world will return!" During the Emperor Gaozong of the Southern Song Dynasty, in order to seize the Blood Netherworld Sword, the Demon Palace on the other side organized a large number of people to go to Jiangxi and bloodbathed the villagers who were said to be responsible for guarding the Blood Netherworld Sword. In the village, local officials dispatched government officials to investigate the case, but they never came back. The officials had no choice but to report it to the court, and the emperor immediately sent four famous arrests to investigate the case. During the investigation process, the four famous arrests and their beloved people went through many hardships, and finally came together. After the seal of Youjian was lifted, Wuqing recalled the memories of his past life, sealed the demons with powerful magic, subdued the Blood Youjian, and the world returned to peace. Wuqing returned to the real world with Sang Zhiyan, and the lovers Iron Hand, Zhui Ming and Lengxue resigned from their positions as agents and retired from the world.