In an old street in the suburban outskirts shrouded in year-round fog, strange occurrences have recently surged frequently at a three-generation legacy old letter house. After closing the house daily, administrator Su Wanhe always discovers a retro kraft paper letter with no sender and no receiving address inside her vacant exclusive mailbox. The contents of the letter are obscure and hard to understand, and the mailbox is repeatedly checked and confirmed cleared before closing every day. At her wits' end, Su Wanhe seeks help by reputation from Shen Yan, a reasoning-skilled writer renting nearby. When the two meet for the first time and preliminarily investigate clues in the letter house's front hall, they find that all doors, windows, and mailboxes show no traces of external forced entry, and the bizarre anonymous letters appear as if out of thin air. A ten-year dust-covered missing person cold case also follows along with the first anonymous letter,