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.