In a deep alley in the French Concession in Shanghai during the Republic of China, Sheng Wanqing, a female English teacher, heard the mournful sound of the flute coming from the empty house next door late at night, and became curious; she met Shen Yan, the detective investigating the murder in the alley, for the first time, and the two teamed up to find the whereabouts of the flute player. , But only a yellowed diary was found; the diary recorded the secrets of Shen Yan's younger brother Shen Che, a deceased underground worker, and unsent love letters; Sheng Wanqing gradually fell in love with Shen Yan during the investigation, but because of Separated by the war; she stayed in the bookstore, inherited Shen Che's legacy, and secretly covered underground activities; Shen Yan returned many years later, his temperament changed drastically, and he was on a mission. The two reunited but misunderstood many times; the truth about Xiao Sheng emerged - the original It was Shen Yan who imitated his younger brother's song to test Sheng Wanqing's loyalty; Shen Yan was seriously injured in the final mission, and Sheng Wanqing stood up to the crisis alone; after the war, Shen Yan woke up and told her ten-year-overdue confession in the Shenxiang Bookstore, and the two finally got married.
