In the small town of Ogden Marsh, Iowa, Sheriff David (Timothy Olyphant) is enjoying a casual baseball game when his deputy, Russell (Joe Anderson), alerts him about a troubling sight: local resident Rory Hamill (Mike Hickman) enters the outfield brandishing a shotgun. When Rory fails to respond, David is forced to take drastic measures and fatally shoots him. As the chilling events unfold, David and his wife Judy (Radha Mitchell), the town's doctor, start to notice increasingly strange behavior among their fellow citizens. Just one night later, a farmer commits an unspeakable act, burning down his home with his family trapped inside.
While investigating the mysterious death of a pilot found in a swamp, David and Russell uncover a submerged cargo plane. As they grapple with the loss of communication, heavily suited soldiers arrive, enforcing a quarantine at the local high school. Here, all residents undergo examinations for signs of infection. Despite Judy's insistence that her fever is due to pregnancy, she is sedated and taken away from David. In a terrifying turn of events, she finds herself strapped to a gurney as chaos breaks out with infected individuals breaching the perimeter.
Desperate, David escapes the facility just as he was about to be evacuated and reunites with Russell. They mount a daring rescue to save Judy and her medical assistant, Becca (Danielle Panabaker), from an infected individual wreaking havoc in the infirmary. After narrowly escaping, the four of them embark on a perilous journey out of town on foot, as they cannot find a working vehicle. Their path intersects with Becca’s boyfriend Scotty McGregor (Justin Miles) on his farm, but tragedy strikes when soldiers raid the premises, leading to his and his mother's untimely deaths.
After they manage to repair a patrol car and hit the road, their efforts are thwarted when an attack helicopter spots them, compelling them to seek refuge in a car wash. Unfortunately, infected employees attack, resulting in Becca's death, and the chopper subsequently destroys the car. Now pursuing their escape on foot, Russell disables a passing government SUV, capturing the driver, suspected to be a government agent (Glenn Morshower). He reveals horrifying details about the cargo plane carrying a biological weapon named "Trixie," which has contaminated the town's water supply.
Fueled by anger, Russell shoots the driver and confronts his companions. However, as he begins to notice his own symptoms, he realizes he is infected. In a selfless act, he distracts the soldiers at a blockade, allowing David and Judy to slip past him. As darkness falls, David and Judy arrive at a truck stop, hoping to secure a vehicle, but they find devastation; all evacuees from their town have been executed. Fighting off three infected individuals, they manage to escape in a semi-truck.
As they flee, the town of Ogden Marsh is obliterated in a massive nuclear explosion, which flips their truck in its wake. Though they struggle onward toward Cedar Rapids, above them, a military satellite monitors the situation, issuing a chilling "initiate containment protocol," indicating that David and Judy are unknowingly infected with the Trixie virus.
During the closing credits, viewers see a news clip from Cedar Rapids Channel 9, where the newscaster (Bruce Aune) reports on the catastrophic explosion originating from the Dakon Petrol chemical plant that ignited a massive inferno in Ogden Marsh. Eyewitnesses describe a towering fireball lighting up the night sky and a subsequent explosion near a truck fueling station. The segment ends with an ominous perimeter being set up, denying civilians access to the area, followed by a soldier with a weapon at the ready and a snarling infected individual, concluding with the ominous phrase "Signal Lost," hinting at the impending spread of the outbreak to Cedar Rapids itself.
