Scientist James Parmiter drops off his wife, Carrie, at a church service at a rural parish before heading to his job at a nearby university. During an intense sermon discussing God's "gentle caress," a devastating earthquake strikes, causing significant structural damage to the chapel, though miraculously, no lives are lost. As the congregation evacuates, Carrie seizes the opportunity to grab her Bible and attempts to call James from a payphone, inadvertently missing her ride home with Henry Tacker and his son, Kenny. Shortly afterwards, as Henry and Kenny make their way to the Tacker farm, tragedy strikes when their pickup truck suddenly bursts into flames. This gruesome scene is witnessed by young Norma Tacker, who is left traumatized by the horrific sight of her father and brother burned alive.
As night falls, Gerald Metbaum ([Richard Gilliland]), Norma's boyfriend, arrives to bring her home from her vigil at the burned truck. Unbeknownst to them, the vehicle is infested with deadly cockroaches. When they return home, Gerald is disturbed by strange sounds and investigates the property, discovering alarming signs of large, fiery cockroaches that have emerged from the ground due to the newly created fault line caused by the earthquake. In a shocking incident, a cat is killed when one of these monstrous insects explodes upon contact with its head.
Days later, Gerald attends a biology lecture conducted by James Parmiter, who teaches that modern humanity has lost its essential ability to communicate with other living beings. Over lunch in the cafeteria, Gerald shares his discoveries at the Tacker farm—now recognized as the earthquake's epicenter—with James, who remains skeptical about the existence of fire-producing cockroaches. To bolster his claim, Gerald presents James with the charred remains of the cat.
Intrigued yet incredulous, James and Gerald visit the Tacker farm, leading to a shocking revelation—these cockroaches are capable of igniting flames by rubbing their legs together. Their alarming behavior is further validated when they observe one of the insects consuming ashes. As chaos unfolds, fires begin to engulf various town structures, prompting James to alert the mayor, who unfortunately is unreachable at that moment.
During a tense afternoon at the university lab, Mark Ross, a colleague of James's, conducts analysis on a cockroach specimen and discovers that these insects are resilient, devouring carbon and impervious to conventional blunt-force attacks. Contemplating their peculiar mode of travel, James is perplexed, as he notes that the cockroaches cannot fly.
At home, Carrie prepares to attend the Tacker funeral alongside Mark's wife, Sylvia, with James arriving just in time to bid her farewell. Left alone, James receives a visit from a handyman, Charlie, who complains of unsettling noises emanating from James’s car. Upon investigating, James uncovers cockroaches devouring his engine and, in an effort to manage the infestation, he uses a freshly burned newspaper as bait.
As the Tacker funeral unfolds, Carrie offers comfort to Norma and her surviving brother, Tom, giving them a handkerchief gifted by Henry earlier that fateful day. Upon returning home, a cockroach hidden in the telephone receiver viciously attacks Norma, compelling her to be rushed to the hospital after Gerald heroically extracts the pest using a burned newspaper.
Meanwhile, James drives to the now smoldering Tacker farm, where his efforts to neutralize another cockroach only result in explosive consequences. His overwhelming excitement leads him to contact Mark, and they convene at the laboratory where James reveals that the insects' slow movements are due to atmospheric pressure, tossing around theories that these ancient creatures could have been trapped underground following a primeval volcanic eruption. Despite the breakthrough, Mark adds to James’s despair with news that the cockroaches cannot reproduce and will self-destruct.
In the days that follow, James persuades Gerald to construct a large pressure tank in hopes of aiding Norma's recovery. At the Parmiter residence, Carrie stumbles upon a surprising birthday cake hidden by Sylvia in the garage, unknowingly bringing doom upon herself as a cockroach clings to the box and subsequently ignites her hair, engulfing her in flames.
In a state of grief, Mark arrives at the lab to find James paralyzed by despair. Seeking closure, James revisits the Tacker farm, where he observes the burnt shells of deceased cockroaches. He captures a surviving cockroach and places it in Gerald's pressure tank, vowing to ensure its survival. His determination leads him to introduce a standard household cockroach into the tank, resulting in an unexpected mating.
As days progress, James becomes isolated, uneasy upon discovering that the cockroaches are reproducing aggressively. When he attempts to return to the Tacker home, he is met with resistance from Gerald, who has learned the whereabouts of James’ hideaway. Ignoring warnings, the couple leaves food for him, but James is tortured by the knowledge of the rapidly breeding creatures he has inadvertently created. After a few more days, James transports the enlarging egg case out of the tank, where it soon hatches vast numbers of cockroach infants maturing at an astonishing rate.
The momentum spirals out of control when James witnesses the adult cockroaches devouring their mother and is left heartbroken. As his emotional and mental stability deteriorates, James awakens to a nightmarish reality where the cockroaches drain his blood while he sleeps. His psyche crumbles under the weight of the insects’ intelligence as he suspects they can interpret his voice, even forming words using their bodies.
At the peak of his terror, he realizes the profound implications of their escape from captivity, leading him to frenzy as he searches for sustenance for the growing horde. The horrifying culmination of events strikes when Sylvia heads to the Tacker residence with a sentimental Bible, only to be met by a nightmare of cockroach infestation. In a chilling encounter, the mutating carnivorous bugs attack her face as she screams, ultimately claiming her life.
Upon returning home, James discovers Sylvia’s lifeless body and, in a rush of panic, tries to contact the authorities, only to face the cold reality of severed phone lines. Hearing the unsettling sounds of the insects, he stumbles outside to encounter the terrifying glow of a fault, momentarily illuminated by the escape of monstrous cockroaches now granted wings. Despite efforts to evade, these relentless creatures persist in their pursuit, culminating in a catastrophic fire leading to James falling into the fault, propelling him and the awakened apocalypse back beneath the Earth’s surface, sealing the cockroaches away once again.
