In an accident, female professor Su Wanhe was involved in an "information exchange" system that spans time and space by an old book with letter paper inside. She can communicate on a limited basis with a person from a specific time in the past (1929). But the rules are extremely cruel: the timeline is not fixed. Every piece of information she writes may change the past, affect the present, and even make her marginalized self disappear. Most of the letters she received were requests for help, help and accusations from that era. In the fog of history, she must use her professional knowledge to identify the authenticity of the letter and help (or stop) a certain person in order to avoid an event that triggers a greater disaster.
