In the town of Ogden Marsh, Iowa, David, the local sheriff enjoys a baseball game. His deputy, Russell, spots Rory Hamill, a local resident, entering the outfield holding a shotgun. David is forced to shoot and kill the unresponsive Rory. David and his wife Judy, the community doctor, begin to notice other town residents exhibiting bizarre behavior. The next night, a local farmer burns his house down with his wife and son trapped inside, killing them.
While investigating the discovery of a pilot's body in a swamp, David and Russell discover a cargo plane submerged under the water. Communication services are lost, and soldiers arrive to take all residents to quarantine at the high school. Everyone is examined for symptoms of infection. Judy does not pass the examination and is separated from David. She explains her fever is due to pregnancy, but is sedated anyway. She wakes strapped to a gurney as the perimeter of the facility is breached by the infected. The military personnel evacuate, leaving Judy and others behind. David is nearing evacuation, but escapes and rejoins Russell. They rescue Judy and her hospital assistant, Becca.