There is a Budu post office in the south of the city. Legend has it that this magical store can send letters to people in any time and space. But people who post letters often have to hand over one of their most precious things in exchange. Fudu, the store manager who runs the store alone, recently recruited two strange new clerks. Then 100 mysterious businesses will follow. What twists and turns and bizarre stories will happen in the process of solving the incident between the heroine Budu, the mysterious new clerk Wangshu, and the two ghostly men? A girl who has lived for a hundred years, a boy with a mysterious background, and an unparalleled and dangerous love. Everyone has their own secrets. Do you also want to mail such a "regret letter"?