Adapted from the best-selling novel of the same name by Japanese mystery queen Miyuki Miyabe, the story tells that in the 1990s, prosecutor Guo Xiaoqi (played by Wu Kangren), who was good at investigating homicide cases, faced the first serial murder case at that time. He used the yardstick of justice in his heart to fight against the serial murderer. Faced with the murderer's constant provocation and the subsequent expansion of evil during the investigation, Xiaoqi determined to dirty his hands and gamble his life to dig out the evidence of the murderer's crime.