In the small town of LaGrange, Ohio, Curtis LaForche experiences increasingly distressing apocalyptic dreams and vivid hallucinations, including visions of rain resembling "fresh motor oil," foreboding swarms of black birds, and threats from those he loves. He conceals these haunting experiences from his wife, Samantha, and their deaf daughter, Hannah, choosing instead to focus his mounting anxiety on a compulsive drive to construct an elaborate storm shelter in their backyard. However, Curtis's erratic behavior – such as cutting ties with those who hurt him in his dreams – creates tension with his family, friends, and employer, ultimately jeopardizing his construction job as he borrows equipment for his project without permission.
To cope with his escalating insomnia and ominous visions, Curtis seeks help from a counselor at a community clinic, delving into his family’s mental health history. He learns that his mother, Sarah, battled paranoid schizophrenia around the same age he is now, leaving him fearful that he may share this fate.
In a reckless bid to finance the shelter's renovation, Curtis secures a home improvement loan that stretches his finances beyond breaking point – all while keeping this secret from Samantha. When she eventually discovers his plans, her anger intensifies. The situation worsens after Curtis takes an excessive dose of sedatives, leading to a seizure that prompts Samantha to call for an ambulance. Once he recovers, Curtis reveals the full truth of his struggles and dreams to her.
As Curtis becomes more absent from work, strains develop with his boss, who ultimately fires him upon learning about the borrowed tools, leaving him with a mere two weeks of medical benefits. This decision coincides with Samantha preparing for Hannah's upcoming cochlear implant surgery, adding pressure to their already strained situation. In a desperate attempt to provide a sense of safety, Curtis purchases gas masks for the family and temporarily extends his previous employer’s health insurance.
Things take a turn when Curtis finds out that his counselor has been replaced, pushing him deeper into despair. Amid the mounting pressure, Samantha insists that they attend a community event to reclaim some normality. However, the evening spirals into chaos when a bitter coworker, Dewart, confronts Curtis and provokes a physical altercation that sees Curtis lash out, proclaiming that a catastrophic storm is imminent and that none of their neighbors are prepared.
Their lives come to a head when a tornado warning forces the family to retreat to the shelter. Upon awakening, Curtis hesitates to remove his gas mask, conflicted by the haunting sounds he still perceives outside. With Samantha urging him that there is no storm, he ultimately flings open the shelter doors, revealing a bright world outside where neighbors are already beginning the cleanup from a passing storm.
Following this, a psychiatrist recommends that the couple take their planned beach vacation, with an emphasis on Curtis seeking help at a facility after the trip. At Myrtle Beach, as Curtis builds sandcastles with Hannah, she signs "storm." Moments later, as Samantha steps outside, the oily rain Curtis envisioned begins to fall, transforming her hand blackened with the ominous rain as she gazes at the menacing storm clouds gathering above the ocean. She and Curtis share a silent understanding as the gravity of their situation looms ever closer, whispering "okay" amidst the looming threat.
