In Ogden Marsh, Iowa, Dr. Judy Dutton finishes paperwork at the local hospital alongside employee Becca Darling while her husband, Sheriff David Dutton, enjoys a local high school baseball game. Dutton’s deputy, Russell Clank, spots Rory Hamill, a resident, entering the outfield with a gun. David tries to get him to drop his weapon but is forced to shoot and kill Rory. David and Judy begin to notice strange behavior in other town residents; they are slow to respond to questions and repetitive in their answers. The next night, Bill Farnum, a local farmer, kills his wife, Deardra and son, Nicholas.
David and Russell investigate a dead soldier who is found in the woods and eventually find a crashed plane that has leaked a toxin into the water supply. Infected residents become extremely aggressive, attacking and killing other people. David tries to convince Judy to leave town but they are captured by soldiers and taken to a temporary military facility at the high school. Judy says that they are being checked for fevers and speculates the infection is a virus. An alert goes off when Judy is examined and she is taken away to be quarantined. She tries to explain that her fever is due to pregnancy, but is ignored and sedated. She wakes up strapped to a gurney as the perimeter of the facility is breached. The military personnel evacuates, leaving her and a room full of infected patients behind, including Becca.