Synopsis：In 1920s Shanghai’s French Concession, Lin Lan, a wealthy young lady newly returned from France, is unexpectedly appointed as a female detective at the Central Police Bureau. She teams up with Qin Xin, a sharp and determined forensic examiner from a humble background, along with the quick-witted officer Dou Mingcong and the humorous streetwise informant Zhao Erbao. Together, they form the Fourth Investigation Unit and embark on a thrilling journey of crime-solving. Though Lin Lan and Qin Xin come from completely different worlds, the two repeatedly crack extraordinary cases side by side, earning a reputation as Shanghai’s legendary “twin heroines of investigation.” Along the way, they gradually confront their own inner wounds and grow into true and trusted friends.