本片讲述了一夜之间二十七具尸体被悬挂当街，且都被抽去了骨头，死相诡谲。向来不信邪的宋慈以一种追求极致真相的态度与女助手雨墨深入调查。通过对死者关系网的盘查，宋慈发现这是起随机杀人案，有价值的是“27”这个数字，这与十年前的波旬教案有关，他向恩师耿知年请教，谁知当天二十七具尸体就在府衙中自燃，虽灰飞烟灭却有迹可寻，宋慈不得不怀疑这一切正和当年负责波旬教案的耿知年有关。与此同时，怀揣着隐秘身世的雨墨也已渐渐接近“真相”，那是她同样无法接受的真相。

This film tells that 27 corpses were hung on the street overnight, and their bones were removed. Song Ci, who has always been unbelieving, conducted a thorough investigation with Yu Mo, a female assistant, in an attitude of pursuing the ultimate truth. Through investigation of the relationship network of the deceased, Song Ci found that this was a random homicide, and the value "27" was valuable. This is related to the ten-year-old tenor case. He asked the teacher Geng Zhinian for advice. The corpse spontaneously ignited in the palace, and although there was no trace of it, Song Ci had to suspect that all this was related to Geng Zhinian, who was in charge of the Boxun teaching case. At the same time, Yu Mo, pregnant with a secret life, is gradually approaching the "truth", which is also an unacceptable truth.