In 1960s rural Japan, the remote town of Ebisugaoka becomes the stage for a psychological horror centered on Hinako Shimizu, a young woman caught between suffocating tradition and her desire for autonomy. Living under the control of her abusive father, who views his daughters as commodities to be married off to settle debts, Hinako resists the passive, obedient future embodied by her mother and her married sister Junko. As pressure mounts for her own arranged marriage to Kotoyuki Tsuneki from a wealthy, fox-worshipping family, her childhood friend Shu Iwai gives her red capsules intended to relieve her tension headaches. These capsules, however, have a far more sinister effect: they allow Hinako to separate into two selves and enter a nightmarish dimension where her deepest fears take physical form.

The familiar town vanishes, replaced by a fog-shrouded landscape of crimson spider lilies and grotesque monsters that caricature womanhood and marriage, from doll-like figures to pregnant, tumor-covered creatures. In this otherworld, Hinako encounters the enigmatic Fox Mask, a figure who guides her through a series of brutal rituals that transform her into his ideal bride, including the severing of her arm for a fox limb and the flaying of her face for a mask. Meanwhile, a spirit inhabiting a doll from her childhood warns her not to trust him. Fox Mask is revealed to be her fiance Kotoyuki, who fell into a coma after a fox attack as a child and awoke possessed by a kitsune deity, becoming obsessed with making Hinako his wife. The struggle between Hinako's two selves, her human resistance and her monstrous, submissive fox bride persona, unfolds against the backdrop of a divine war between the kitsune Kyubi and the rival spirit Tsukumogami, which fuels the chaos. Her circle of friends becomes entangled in this nightmare. The jealous Rinko Nishida, who harbors a complicated obsession with Hinako masked by her crush on Shu, transforms into a volcanic priestess monster spewing hatred. Sakuko Igarashi, who felt betrayed by Hinako's engagement, also haunts the otherworld as a projection of abandonment. Shu himself is not innocent; his desire to keep Hinako from growing away from him led him to give her the capsules, making him a vessel for Tsukumogami in some narrative paths.

The story culminates at Hinako's wedding altar, where the conflict between her inner and outer selves reaches its breaking point. Multiple outcomes are possible. In the default ending, her inability to reconcile her identity leads to a psychotic break, and she massacres her groom and Shu with a knife before fleeing in her bloodstained wedding dress. The good ending sees her reject Fox Mask and escape the otherworld with Shu, choosing an uncertain but free future. The bad ending results in her monstrous fox self fully consuming her human identity, transforming her into a docile, perfect wife while her terrified true face is left discarded and crying at the bottom of stairs. Only in the true ending does Hinako find peace. Both versions of herself reconcile after defeating the deities that possessed them. She and Kotoyuki, freed from his curse, decide to live life on their own terms first, allowing their feelings to grow naturally without coercion, finally granting Hinako the autonomy and silence to decide her own future.