Cui Buqu and Feng Xiao have thwarted the corrupt officials embezzling grain in Guangqian County, but the Thirteen Floors of Yunhai and their elusive leader, Xiao Lü, remain at large. As Feng Xiao recovers from the battle, he finds his relationship with Cui Buqu on new footing—though Cui Buqu will be last to admit it.

Yet on their way back to the capital, a strange new case leaves them both at a loss: The entire household of a villa has been brutally massacred, and there’s no sign of the murderer. The only traces they find point to an arcane poison—one that, if deployed in the Sui capital, could put the empire itself at risk. Yet time is running out for Cui Buqu, whose precarious health continues to deteriorate. To get to the bottom of this final case, he must put his very life on the line in the investigative duo’s greatest trial yet.