In a harrowing adventure set 1,000 feet below Pennsylvania's Appalachian Trail, a group of cavers inadvertently stumbles upon a vast chamber teeming with prehistoric bats. As the bats become disturbed by loud noises, they begin to swarm the area, ultimately becoming known as the 'vesps.'
In Montclair, N.J., just 20 miles northwest of NYC, Hugh Andrews, along with his wife Kelly, their deaf daughter Ally, their younger son Jude, and Kelly's mother Lynn, soon become aware of the dangerous vesps through a frantic television broadcast detailing the chaos and death that the epidemic is causing globally. Seeking refuge from the turmoil, Hugh’s best friend Glenn decides to join the family as they set out in two vehicles with Ally's dog, Otis, on a desperate journey to safety.
While on their way, Ally connects with her crush, Rob, via FaceTime to discuss the escalating crisis. A dangerous encounter at a gas station leads to Glenn shooting an armed man attempting a carjacking. However, the group soon faces an unexpected traffic jam, forcing Glenn and Hugh to navigate off the beaten path. In a moment of panic, Glenn swerves to avoid a stampede of deer, resulting in a catastrophic vehicle crash, which leaves him trapped.
In a tense moment, Hugh must continue without Glenn, who distracts the swarm of vesps by sacrificing himself. With his family inside the vehicle panicking, Hugh is compelled to let Otis go, whose barking attracts more vesps. When Lynn's coughing draws the attention of a vesp towards them, Hugh quickly throws a tire iron to divert the creature. In a desperate act of survival, he ignites Glenn's wrecked vehicle to draw the vesps into a fiery trap.
Stranded and unable to use their vehicle due to the noise it makes, the family decides to hike on foot into the forest. They come across a fenced cabin compound, but things take a turn for the worse when the elderly owner is attacked and killed as she tries to shoo them away with a shotgun. The family finds a way through the fence via a drain tunnel, where they face another attack from the vesps, resulting in Kelly being bitten by a rattlesnake.
Inside the cabin, as they tend to Kelly's wounds, Ally manages to contact Rob again, only to learn that their situation is deteriorating. Lynn expresses concern about Kelly’s infection, prompting Hugh and Ally to risk a journey to a deserted town to gather medical supplies. Along the way, they meet a mysterious man known as 'The Reverend,' who offers them a cult-like invitation to join 'The Hushed.' They refuse, only to later discover that the Reverend has sinister intentions regarding Ally.
As tensions mount, a little girl appears at the cabin, but soon it becomes apparent that she is connected to The Hushed, revealed through her missing tongue. The girl inadvertently triggers a swarm of vesps when her collection of ringing cellphones attracts the creatures, leading to a chaotic struggle. In the pandemonium, members of The Hushed attempt to abduct Ally.
A climactic battle ensues, resulting in Lynn sacrificing herself to save Ally, drawing the vesps away while Hugh and Kelly fight off the kidnappers and ultimately kill the Reverend in the process. Now, with their resolve hardened, Hugh, Kelly, Ally, and Jude press on northward until they finally reach 'The Refuge.' Here, Ally reunites with Rob, and together they begin developing their skills to hunt the vesps, determined to survive in a world turned upside down by chaos and fear.
