Qingchuan Town is surrounded by mountains and dense fog. Its people depend on the "mountain springs in the mist" for their livelihood and abide by one rule: never enter the mountains on a foggy day. But after the first heavy fog of autumn this year, Chen Jingshan's daughter suddenly disappears, leaving behind only a pair of shoes stained with muddy spring water.
Shen Zhixia, an outsider and mystery writer staying in town, had hoped to draw inspiration from Qingchuan's fog for her novel. But Chen Jingshan's plea for help pulls her into the case. As she investigates, she discovers that the town has been hiding a string of "foggy-day disappearances": a herb gatherer who vanished twenty years ago, an elementary school teacher who disappeared ten years ago, and even a tourist who went missing six months ago, all vanished without a trace as if swallowed by the mist.
Stranger still, the townspeople refuse to discuss the disappearances, especially the family of "Mist Keepers" who oversee the mountain springs. They keep trying to obstruct her investigation. When Shen Zhixia follows the clues to an abandoned waterworks in the mountains, she finds half a piece of clothing belonging to Chen Jingshan's daughter at the bottom of the reservoir. The old slogan on the waterworks wall is identical to one recorded in the notes from the scene of the herb gatherer's disappearance twenty years earlier.
The fog grows thicker, and Shen Zhixia realizes that Qingchuan's mist hides not only the whereabouts of the missing, but also a dark secret the entire town is protecting.
