There has been a lot of rumors. It's almost mid-autumn festival, according to the customs, the Crown Prince is going to mourn his deceased mother by the river in the Royal Garden. Guo Changzhi, the Royal Chancellor, is worried about his safety and orders the Da Lisi to solve the case as soon as possible. The Da Lisi, together with Xue Yang, Shangguan Yu and Ding Nai'er, find the murderer behind the case and discover that a huge flesh-eating fish demon which breeds under the Qu River. This fish demon is actually owned by Wu Yantuo–a mysterious yet powerful witch. Because of an earthquake, the fish demon escaped from the cage, which caused the tragedy to occur. The reason for that is actually for the fish to eat up the Crown Prince who was about to become the Emperor. Because this has something to do with the succession to the throne, Xue Yang was vilified and imprisoned. Fortunately, he was rescued by Guo Changzhi and had men secretly sent to assist him. On the mid-autumn night on the Qu River, a man-fish battle stops the plans to murder the Crown Prince....