In Pierce County, Iowa; the sheriff is forced to kill the local Rory Hamill who suddenly threatens the community at a baseball game with a shotgun. The sheriff's wife Dr. Judy examines another local, Bill Farnum, who is also exhibiting a strange behavior. That night, Bill traps his wife and son in their home and then burns it to the ground, killing both. The following morning, the sheriff and his deputy are called by three hunters who have discovered a dead pilot in the nearby Hopman Bog. The sheriff and deputy find a large airplane at the bottom of the bog. The army suddenly seals off the town and imprisons the population in tents and concentration camps. The sheriff, his deputy, his wife and her medical assistant, escape. They quickly learn that the plane was airborne with a biological weapon and crashed into the bog contaminating the water supply of the population. To make matters worse, there is no antidote for the victims who are doomed to die or become incurably mad. The quartet tries to find a breach in the containment to reach the next town, but the escapees are hunted by the army.