In the third year of Jinghe, undercurrents surged in the Gilded City of Dayin Dynasty. This capital city, famous for its "Golden Scale Pond", is a bustling scene during the day when merchants gather, but at night there are 72 underground rivers rushing under the city walls - these waterways not only transport the entire city's sewage, but also spread the legend of the "Water Ghost City". Every night when the moon dims, the memory of the deceased will condense into golden scales and turn into human form in the dark river to revisit their hometown. When six-door agent Lu Huaisha was investigating a series of drowning cases, he discovered that all the deceased had gill-like gold patterns on the backs of their necks. When he followed the clues and found Xuan Ying, a mermaid girl who could read memory scales, he saw the figure of his late mentor in a vision of an old case from twenty years ago...
