Story of Ming City Wall

In order to benefit the people, the wealthy Shen Fu of the Ming Dynasty paid for the construction of the city wall. Unexpectedly, during the construction process, the river demon rampantly destroyed the city wall under construction, and the people were panicked for a while. In order to appease the hearts of the people, the emperor ordered Shen Fu to present a cornucopia of heirlooms to suppress Xie Chong, and within a three-day period, he ordered Shen Fu and Li Han Yang to repair the city wall. The river demon continued to commit crimes, and Shen Fu and his companions investigated together and discovered that there was something hidden in the matter.
What kind of shocking secret is hidden behind this? While fighting the monsters, how did Shen Fu solve the case within the three-day deadline?
(Source: Douban)