Plot

In 1930s Oklahoma amid the region's horrific dust storms, a woman is convinced that a sinister presence is threatening her family.

In 1930s Oklahoma, Margaret Bellum lives on a formerly successful farmland, now razed by the Dust Bowl. She works alongside her two daughters, teenager Rose and her younger sister Ollie, who became deaf and mute after a bout of scarlet fever. Her youngest daughter, Ada, died due to the illness, causing a grieving Margaret to experience violent sleepwalking episodes, which she prevents by taking sleeping pills. The family patriarch, Henry, has moved away to Philadelphia to take a construction job, and though he wanted Margaret and the girls to join him, Margaret declined due to her unwillingness to leave Ada's grave behind. One night, Rose reads "The Grey Man" to Ollie, the story of a man who kills his family, but is overtaken by a dust storm and perishes; he can enter victims' bodies and control them.

During a weekly sewing circle, Margaret learns that a drifter allegedly broke into a neighbor's house, tied up the father, and murdered the rest of the family. She becomes more anxious while protecting the property, and her fears become true when a strange man, Wallace, is discovered hiding in her barn. Wallace, a preacher, is wearing Henry's coat and claims that he encountered Henry on the job site, and the latter requested that he pass by to check on the family; he had only been hiding to recover from the storm. A suspicious Margaret orders him to leave, but when Wallace miraculously heals Rose's dust-induced bloody nose, and then promises to help the family's ailing cow, Margaret relents and allows him to stay.

The family's situation improves thanks to Wallace's presence, and the area even enjoys a rain shower, which Wallace had promised to request from God. Eventually, a letter from Henry reaches the family after many dust-related delays. Margaret is shocked when Henry unwittingly reveals that Wallace had stolen the coat and money therein, and possibly murdered another worker. Margaret forces him to leave at gunpoint. Wallace burns the letter so that Margaret cannot bring it to the sheriff, and warns Margaret that he will be back and cannot be stopped, since he is "The Grey Man."

Margaret takes his warnings seriously, even though Rose insists that she had lent him the story, and that he is just trying to intimidate her. Margaret moves the family into one room, seals the house, and gradually stops leaving the house. She also stops taking her sleeping pills in order to stay alert for Wallace, which causes her to hallucinate and sleepwalk. Despite the house's security, strange events begin to occur, such as the girls' bed getting lit on fire during the night and objects moving despite all doors being locked. Margaret's paranoia increases even though later gossip at the sewing circle indicates that there might be no murdering drifter, and the killer might instead have been the family patriarch.

Margaret eventually takes the girls to a local dance on the advice of her sister, Esther, as it is rumored around the area that she is going insane, and Margaret is afraid that she will be deemed unfit to raise the girls and they will be taken away from her like Ada was. However, at the event, Margaret has a public breakdown, claims that Wallace is a paranormal entity that is attacking her household, and warns the congregation of "The Grey Man." Rose takes her back home, where Margaret has a hallucination of Wallace invading the property and shoots his corporeal self. In the process, she nearly kills Rose, who begs her to wake up; Rose later discovers that Margaret shot not Wallace, but Esther, who had visited to check on Margaret. Rose tries to escape with Ollie, but a dust storm is rolling in, leaving them trapped.

As the storm begins, the sheriff arrives, as he had been concerned about Margaret's demeanor since the dance. A resigned Rose admits Margaret is unfit, but an insane Margaret stabs him to death before he can take the girls away. She tells Rose that since they will execute her for killing him, they all must commit group suicide so they can stay together and join Ada. Unable to kill her mother outright, Rose instead tricks her into going out into the storm. She tearfully cuts the safety line, leaving Margaret to die in the storm. The girls later travel to Philadelphia to reunite with their father.