Iris Blum, a former horror author grappling with dementia, resides in a secluded home in Braintree, Massachusetts. This eerie residence was constructed by a man for his new bride, but on their wedding day, both the bride and groom mysteriously vanished, leaving the house tragically unfurnished. To provide care for Iris, her estate manager, Mr. Waxcap, hires live-in nurse Lily Saylor.
On her very first night at the estate, Lily experiences an unsettling encounter when an unseen force violently wrenches the telephone from her hand, and she catches a glimpse of a figure dressed in white, walking backward. As time passes, a disturbing spot of black mold appears on the wall, growing ever larger, while Lily also notices that a corner of the rug at the base of the staircase has been mysteriously flipped up—curiously, she is the only one traversing the first floor.
Throughout her time with Iris, the former writer refers to Lily as "Polly," a name that Mr. Waxcap explains belonged to the main character in Iris's most renowned novel, The Lady in the Walls. When Lily delves into a copy of the novel, she uncovers unsettling implications—that Iris had a connection to Polly and may be recounting a forgotten tale from her own past. The story takes us back to 1812, where Polly, clad in a wedding dress and blindfolded, wanders through the abandoned house, unaware of the sinister events unfolding above her.
In a chilling episode within the kitchen, Lily hallucinates that her arms are now grotesquely bloated and speckled with black mold. Later that evening, she spots a reflection of the white-clad figure but is met with empty air when she turns around. Her curiosity drives her to explore a hidden cardboard box in a closet, where she discovers rough drafts for The Lady in the Walls. This revelation prompts her to suspect that the narrative may not just be fictional, but rather an ominous recounting of a murder that transpired in the home.
As the scenes shift back to 1812, blindfolded Polly, relying on her sense of touch, stumbles upon a wall stripped of wood—remarkably, the same area now tainted with mold in the present day. When Polly dares to lift her blindfold, she locks eyes with her husband, feeling a sense of dread as her husband suddenly strikes her down with a hammer, concealing her lifeless body behind the wall.
In the present, Lily attempts to engage Iris in conversation about the book, only to be met with fury as Iris accuses Polly of betrayal and abandonment, reminding her that even the most beautiful things succumb to decay. A chilling encounter unfolds as Polly’s ghost appears to Iris, whispering dark secrets. In her desperate quest for understanding, Lily discovers the wall boards have been removed and heaped next to the mold-infested wall, only to confront the apparition of Polly, leading to her sudden demise from a heart attack.
Days later, Mr. Waxcap stumbles upon the grim scene, uncovering both Lily's and Iris’s lifeless forms. Years roll by, and a new family moves into the haunted house, now under the watchful eyes of Lily's ghost, forever intertwined with the tragic history of the place.
