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Director Ma Songtian of the Yanjiang City Public Security Bureau is preparing to select one of the two outstanding police officers, Yang Xin (forensic doctor) and Li Yun (trace examiner), to be the captain of the criminal police brigade. Originally, Yang Xin's excellent reasoning ability made the captain position a sure thing, but she let the real murderer go while handling a case and was demoted and transferred to the police station. Until a poisoning case of a vicious nature appeared. Director Ma personally promised to solve the case and let her return to the team. The murderer hiding in the dark will have no way of hiding