In the heart of 1930s Oklahoma, Margaret Bellum finds herself on a farm once filled with promise, now devastated by the merciless Dust Bowl. She tirelessly cares for her two daughters: the teenage Rose and her younger sister Ollie, who communicates through silence after losing her hearing and voice to scarlet fever. The tragedy of losing their youngest daughter, Ada, weighs heavily on Margaret, leading to haunting sleepwalking episodes that she tries to suppress with sleeping pills.
The family's patriarch, Henry, has relocated to Philadelphia for work, hoping the rest of his family will join him. However, Margaret's grief for Ada's grave keeps her rooted in place. One evening, during a storytelling session, Rose reads a chilling tale titled The Grey Man, about a man who kills his family only to be claimed by a dust storm, allowing him to inhabit the bodies of his victims.
Amidst their struggles, unsettling news arrives at the weekly sewing circle: a drifter has committed horrific crimes against a neighboring family. This revelation heightens Margaret's fears until one fateful night, when a stranger named Wallace is discovered hiding in the barn. He claims to be a preacher who was sent by Henry to check in on the family, having sought shelter from a storm. Upon witnessing Wallace's miraculous healing of Rose's injury and his willingness to assist their suffering cow, Margaret reluctantly allows him to stay despite her initial suspicions.
With Wallace's arrival, a sense of hope seems to rekindle, accompanied by the first rain in ages, which he promised as part of his divine intervention. However, a letter from Henry shatters this newfound peace, disclosing Wallace's dark secrets of theft and potential murder. Feeling betrayed, Margaret confronts Wallace, forcing him to leave, but he responds by burning the letter and issuing chilling threats about his return, calling himself “The Grey Man."
Margaret, caught in a web of fear and suspicion, confines her family to a single room and gradually becomes a prisoner of her own making, neglecting her sleep aids to remain vigilant against Wallace. What follows is a series of eerie incidents—mysterious movements within their home and even a fire that ignites in the girls' bedroom—leading Margaret to doubt her own sanity despite sewing circle gossip suggesting that the supposed drifter may be just a figment of her troubled mind.
On her sister Esther's advice, Margaret finally attends a local dance to confront rumors about her mental state. This outing spirals into chaos as she publicly accuses Wallace of being a supernatural threat and implores her community to recognize the danger he poses. This outburst triggers a subsequent series of hallucinations, culminating in a tragic mistake: the belief that she has shot Wallace when, in reality, she harms Esther.
Left in despair, Rose tries to flee with Ollie but is caught in an impending dust storm. Complications arise as the sheriff arrives, aiming to help, only to be met with a frantic and ultimately fatal confrontation with Margaret. She, sensing her fate has been sealed, proposes a harrowing plan for the family to reunite with Ada in the afterlife.
In the end, Rose’s love for her sister and need for survival lead her to take a desperate measure, tricking Margaret into facing the storm alone, and ultimately allowing her to perish. The surviving girls then set off towards Philadelphia to seek their father, leaving behind the remnants of a tragic and haunting past.
