Scientist James Parmiter (Bradford Dillman) delivers his wife, Carrie (Joanna Miles), to a church sermon at a country parish, then drives to work at a local university. During a sermon about God's "gentle caress," a major earthquake strikes and nearly demolishes the chapel, but no one is killed. While the parishioners evacuate, Carrie grabs her Bible and calls James from a payphone, thereby missing her ride home with Henry Tacker and his son, Kenny. As the two men approach the Tacker farm, their pickup truck explodes in flames. Young Norma Tacker (Jamie Smith Jackson) watches in horror as her father and brother burn.That night, Norma is retrieved from her vigil inside the burned truck by her boyfriend, Gerald Metbaum (Richard Gilliland). Both are unaware that the vehicle is hosting killer cockroaches. Returning home, Norma retires while Gerald hears strange noises and surveys the property to discover large, incendiary cockroaches near a fault line created during the earthquake. A cat is killed when a cockroach com-busts on its head.Sometime later, Gerald visits the biology class of his former professor, James Parmiter, who lectures that modern man has lost his innate ability to communicate with other living creatures. When the students leave, the two men eat lunch at the cafeteria and Gerald reports his entomological discovery at the Tacker farm, the earthquake's epicenter. When James argues that fire-producing insects do not exist, Gerald shows him the dead cat.James goes to the farm with Gerald and is astonished that the cockroaches are able to make fire by rubbing their legs together. He also observes one cockroach feeding on ashes. Soon, the town is besieged by fires in most of its buildings and James calls the mayor to warn him about the bugs, but the mayor is unavailable.Back at the university laboratory, James's colleague, Mark Ross (Alan Fudge), examines a cockroach specimen and reports that the insects eat carbon and are indestructible to any blunt force. James wonders how the cockroaches travel from place to place, since they cannot fly.At the Parmiter home, Carrie prepares to leave for the Tacker funeral with Mark's wife, Sylvia, and James returns in time to say goodbye. Alone in the house, James is visited by a handyman named Charlie, who reports strange noises coming from James's car. James discovers cockroaches feasting on his engine and orders Charlie to leave. He then lures the insects into a trap with a freshly burned newspaper.Meanwhile at the Tacker funeral, Carrie gives Norma and her surviving brother, Tom, a handkerchief that Henry gave her at the church just before the earthquake struck. When Norma, Tom, and Gerald return home to pack up the house, Norma answers the telephone and a cockroach, nesting in the receiver, attacks her ear. Gerald extracts the bug with a burned newspaper and Norma is sent to the hospital.Meanwhile, James drives to the farm, which is now smoking from the fires triggered by the cockroaches. He impales one of the insects, causing it to explode. Excited by his discovery, James phones Mark and they meet at the laboratory. James explains that the bugs are slow-moving because of atmospheric pressure, and they are suffering from decompression sickness. Before the earthquake, the cockroaches lived entirely underground in a dense atmosphere, but now they are struggling to stay alive. James suspects the insects are ancient creatures trapped underneath the Earth's surface during a prehistoric volcanic eruption. When Mark adds that the cockroaches cannot reproduce and that they will soon self-destruct in the Earth's atmosphere, James is despondent.Sometime later, James asks Gerald to build a large pressure tank. Gerald hopes that James' experiments will help Norma recover from the cockroach attack and agrees. Back at the Parmiter home, Carrie discovers a surprise birthday cake that Sylvia hid in the garage. She brings it inside, unaware that a cockroach is clinging to the box. The insect attaches itself to Carrie's shirt, then sets her hair on fire, causing the woman to ignite.Later, Mark arrives at the laboratory to find James paralyzed with grief. Seeking to make sense of the tragedy, James returns to the Tacker farm and sees the burned shells of deceased cockroaches. He captures a sole surviving insect and places it in Gerald's newly crafted pressure tank, muttering, "I will make you live." He then introduces an average household cockroach into the tank and the two bugs copulate.Sometime later, Gerald and Norma return to the Tacker home, but he refuses them entry. James is angered to learn that the location of his hideout has been leaked to Mark. The couple leave food for James, then drive away. Days pass and the cockroach mating is successful. However, the female "fire bug" guards her egg case and James is unable to perform experiments.Several more days later, the egg case increases in size and James transfers it out of the pressure tank and into a box. There, the egg hatches numerous infants that grow to adult cockroaches within five hours. When James reintroduces the mother cockroach to her offspring, they kill her and James sobs. Soon, cockroaches escape from their box and feast on James's uncooked steak; he discovers that they only consume raw meat and eat collaboratively, as a "single unit." That night, James awakens to find cockroaches sucking blood from his chest and secures them in the pressure tank.Over the next several days, James suspects that the cockroaches listen to his voice and watch him, as well as form symbolic patterns with their bodies to communicate messages. Late one night, he discovers that the insects have escaped the tank to form the letters of his last name on the wall, along with the message: "We live." Terrified, James realizes the bugs have great powers and runs from the house to get them more food. Meanwhile, Sylvia takes a taxi to the Tacker home, intending to give James the Bible that Carrie held so dear. She screams upon finding the house infested with cockroaches and a cassette tape of James' voice, repeating: "I've gone too far." As the insects burn her face, Sylvia runs outside and dies in the barn. Feasting on their kill, the bugs procreate and climb into the fault line. When James returns, he discovers the insects missing.That evening, Mark arrives, looking for Sylvia, and notices that James has become mentally unhinged; unshaven and disheveled. Although James orders his colleague to leave, he promises to return to the university in a few days. With Mark gone, James is horrified when he discovers Sylvia's body. He grabs the Bible and tries to phone the authorities, but the line has been severed by the mutant cockroaches. Hearing the insects, James wanders outside to see a bright red light emanating from the fault as giant cockroaches, now equipped with wings, fly away. Although James tries to escape into the house, the creatures smash through the window in pursuit and set him and the whole house on fire. As James tumbles out of the front door in flames, he falls into the fault and the cockroaches follow. The fissure closes, trapping the cockroaches underneath the Earth's surface yet again.