The film mainly tells the story of a sudden water monster cannibalism incident in Qujiang Lake in Chang'an City. Dali Temple Minister Xue Yang (played by Li Yan) and coroner Shangguan Yu (played by Xu Dongdong) were ordered to find out the truth of the murder. During the investigation, the two found that there were a large number of scattered skulls and human bones at the bottom of Qujiang Lake, among which was hidden a shocking conspiracy concerning the stability of the court. An unknown body died tragically in the river. In Chang'an City, rumors of fish monsters were spreading. Several little beggars disappeared strangely one after another, which made the case more suspicious. Was the fish monster deliberately raised by the monster Wu Yantuo (played by Yin Ziwei)? The prince (played by Ma Lu) was besieged by the fish monster and the killer when he offered sacrifice to his mother in Qujiang Lake? Why did the mysterious female swordsman (played by Peng Jing) risk her life to hunt down Wu Yantuo? Who is loyal and who is treacherous between the Imperial Censor Guo Changzhi (played by Liu Fenggang) and the Minister of War Shen Shu (played by Yang Yongwen)? The thrilling cases are linked together and the mystery is about to be revealed.