On a quiet night, a meteorite crashes into Wheelsy, a small town in South Carolina, carrying a malevolent, sentient extraterrestrial parasite. The organism quickly finds a host in Grant Michael Rooker, a wealthy resident, taking over his body and mind. Under its control, Grant begins a disturbing transformation, becoming a grotesque, tentacled monster that exudes a dangerous presence. The parasite also abducts and infects a local woman named Brenda, using her as a breeder for its growing larvae.
Back at home, Starla Elizabeth Banks, Grant’s wife, grows suspicious as she notices changes in his appearance and behavior. She wonders why he hasn't set foot in a doctor's office in over a year, and her concern deepens when Grant attacks her. The danger escalates until the police arrive to intervene, at which point Grant flees into the night.
Meanwhile, a posse led by police chief Bill Pardy Nathan Fillion tracks the alien’s trail and discovers Brenda's body, which has become unnaturally swollen from the larvae inside. The larvae burst out, spreading rapidly and infecting most of the town, sparing only Starla, Bill, Mayor Jack MacReady, and teenager Kylie Strutemyer. The infected townspeople become part of a hive mind, all under Grant’s direction, bent on consuming every life form until only Grant’s consciousness remains.
Despite the invading menace, Grant clings to a stubborn thread of love for Starla and wants to be reunited with her. The survivors realize their best chance is to kill Grant, which would in turn destroy the rest of the aliens. Bill and Kylie manage to escape with Starla and Jack briefly captured, but the group is fractured.
Armed with a grenade, Bill and Kylie head toward Grant’s house, where the infected are absorbed by the increasingly mutated host. Inside, Jack and others are converted into breeders for more larvae while Grant keeps Starla uninfected for the moment, hoping to win back her affection. Starla, resourceful and determined, arms herself with a sharp brush and confronts Grant in a tense showdown.
She tricks him into believing she still loves him, then stabs him with the brush. Grant becomes furious and throws her across the room. Bill and Kylie enter, shooting several infected townsfolk and grabbing a grenade. Grant knocks the grenade into the pool, then subdues Kylie with a couch and attempts to infect Bill with his tentacles. While one tentacle stabs Bill in the stomach, Bill attaches the other tentacle to a propane tank. Starla shoots Grant, who, filled with flammable gas, explodes, killing the rest of the aliens and saving Bill. With the town dead and the threat eliminated, the three survivors head off to seek help.
In a post-credits scene, a cat approaches Grant's remains and becomes infected by the alien parasite, a quiet reminder that the danger may not be entirely past.
