In Suzhou in 1935, Chu Yu'an, a declining scholarly girl, sent a letter asking for help at the abandoned concession post office in order to find out the truth about her father's disappearance. Little did she know that this letter spanned a two-year time difference and arrived in Shanghai on the eve of the Anti-Japanese War in 1937, falling into the hands of Xiao Ling, the inspector of the French Concession.
The two communicated through the air through a mysterious mailbox, neither knowing that they were separated by two years. Chu Yu'an's "Future Reminder" turned out to be a misunderstanding, but accidentally touched the clues of the murder case that Xiao Lingdong was pursuing; Xiao Lingdong mistook her transparency for psychic, causing misunderstandings and secret feelings.
As the communication deepens, the secret of Chu's father's disappearance, the oracle bones coveted by the Japanese army, and the Japanese secret service disguised as a cultural association are gradually intertwined. They investigate the case side by side in the gap between time and space, but they are trapped in the fateful dilemma of "predictability but cannot change".
When Chu Yuan went to Shanghai for an appointment with the secret message, Xiao Lingdong searched frantically in 1937, only to realize that the gap in time and space was insurmountable. The bell of the Marco Polo Bridge Incident is approaching, and personal love cannot withstand the torrent of history. A rusty mailbox carries unspoken love, freezing the regrets and concerns of the two eras.
