During the Republican era, a series of bizarre poisoning murders occur in Liangcheng. Police Captain Ke Zeming joins forces with forensic doctor Su Qingyuan, wealthy young lady Cheng Siyue, and his mentor Chen Yifeng to investigate the truth, only to discover a smuggling network trafficking poisonous substances from Southeast Asia behind the cases. As the investigation deepens, Ke Zeming gradually realizes that there is a mole within the police station. The seemingly gentle Police Chief Xia Xianqi turns out to be a Japanese spy who has been undercover for twelve years. After enduring numerous dangers, the group finally exposes the conspiracy and captures the real culprit, but a new mystery remains.