Ed Funsch is a seemingly ordinary guy who performs a mundane job at the United States Post Office in Franklyn, Pennsylvania. One day his supervisor delivers bad news. Because of cutbacks, Ed has been laid off. Depressed about his prospects for the future, Ed nonetheless returns to his spot at the zip code sorter machine. As he stares at the digital display on the device, numbers begin changing into letters of the alphabet to form words. Instead of seeing postal codes, Ed views the message, "Kill 'Em All."

Shortly thereafter, a real estate agent enters a crowded elevator in an office building. He begins sweating profusely, and as he stares at the elevator's digital display floor-indicator, he too sees the message "Kill 'Em All." The man loses control. He strikes out at the other passengers, killing two in a wild frenzy.

The senselessness of the crime, and its similarity to other violent incidents that have rocked the small town of Franklyn, prompts local police to ask for the FBI's help. Mulder is assigned to the case. He learns that the real estate agent and others who suddenly went berserk did so in busy, public areas. None of the suspects obeyed orders to surrender and were subsequently shot to death by police. Mulder suspects that electronic devices found smashed at the scenes of the crimes are linked to the killings.

Another bizarre killing shakes the small town when a woman attacks a lecherous automotive mechanic at the garage where he works. Mulder inspects the murder scene and is most intrigued by a smashed digital dashboard display. Accompanied by local police, Mulder interviews Bonnie McRoberts, whose car the mechanic was working on at the time he was murdered. McRoberts suddenly goes berserk and slashes at Mulder with a knife. Police open fire, killing her. Scully examines the woman's corpse and finds evidence that a chemical similar to LSD is present in her blood. That chemical, however, is only triggered when a high amount of adrenaline is secreted.

Mulder discovers firsthand that the city of Franklyn sprays orchards with dangerous insecticides in its quest to exterminate a particularly virulent species of cluster fly when he is doused by a helicopter. County Supervisor Larry Winter defends the operation, noting that the local economy depends on crops.

Mulder's exposure to the chemical begins to have an effect on his mind. He too sees messages in electronic displays. He realizes that all of the killers suffered from phobias that could be exploited through subliminal means. Since the chemical agent used in the manufacture of the insecticide produces fear in cluster flies, Mulder concludes the same fear is being chemically induced and then triggered by someone using the town of Franklyn for a controlled experiment.

Mulder and Scully believe that only a small percentage of the population of Franklyn - some 25 people - are at risk of having a violent reaction to the chemical. One of these is Ed Funsch. The agents arrive at Funsch's home and discover an empty rifle case. Mulder realizes that Funsch is afraid of the sight of blood. He races to a local blood drive at a college, where Funsch has climbed a clock tower armed with rifles. Mulder overpowers Funsch and arrests him.