At the end of the Qing Dynasty (1636-1912) and the beginning of the Republic of China (1912-1949), the wealthy Lin family is destroyed overnight. The only surviving daughter, Lin Mengchen, is sold to the wicked wealthy Huang family, to marry their deceased son in a ghost wedding. Just as Mengchen is about to be buried alive, Xiaosheng, the Blood Sacrifice Hall's top assassin who has been tasked to wipe out the Huang family, accidentally comes to her rescue. Witnessing the entire process of Xiaosheng killing someone, not only does Mengchen show no fear, but she also regards the man on the mission as her benefactor. Xiaosheng then decides to keep Mengchen by his side. They address each other as brother and sister, with Xiaosheng vowing to protect Mengchen for the rest of his life. Ten years has passed in the blink of an eye. In order to let his "sister" grow up in a good environment, Xiaosheng has been living with a dual identity. In front of Mengchen, he is Mr. Ye, a businessman engaging with foreign firms. In the hidden darkness unknown to Mengchen, he remains the top assassin of the Blood Sacrifice Hall. Yet, little does Xiaosheng know that Mengchen, this innocent and naive sister of his whom he has protected for ten years, has in fact found out about his true identity and secretly joined the Blood Sacrifice Hall. She has become Xiaosheng's disciple. She is now an assassin in the making named Guyu...