On the remote island town of North Port, the sheriff Franc Luz Richard Tarbell wakes to a troubling morning when he finds several cockroaches in his house. He drives to the airport to greet Elizabeth Johnson, Lisa Langlois, the daughter of mayor Robert Lansing Elias Johnson and a former girlfriend of Tarbell, returning after a four-year absence for her father's birthday. Strange incidents ripple through the town: the bindings of every book in the library are damaged, and several dogs lie dead in bloody carcasses.
Terri Treas as Dr. Morgan Hubbard arrives to investigate the dog deaths. She sets a trap with a live cat as bait; cockroaches swarm to it and devour the cat. While Hubbard studies the insects, Elizabeth visits the local diner, owned by Tarbell’s current girlfriend, Lillian, Nancy Morgan. Stephen Davies as Homer, the town's pest-control man, attempts to exterminate cockroaches inside the diner.
Hubbard notes that the cockroaches reproduce asexually, and her own hand is bitten by roaches. She sprays rotenone, but only a dosage lethal to humans will stop them. Elias suggests evacuating the island, but Hubbard urges him not to. Meanwhile, Tarbell shows a number of small objects found on a dog carcass to Homer, who identifies them as Periplaneta droppings. Richard’s secretary Millie informs him that Hubbard previously worked in genetics at MIT and conducted illegal experiments.
Elizabeth ventures into a cave and uncovers equipment belonging to INTEC, a corporation with which Elias has a deal to develop the island. After being chased from the cave by flesh-eating cockroaches, she and Tarbell confront Elias and Hubbard. When Elias demands that Tarbell cease his investigations, Tarbell resigns as sheriff. Several people are killed by the cockroaches, including diner worker Jeff Winkless as Church, Homer’s friend Jack Collins as Shakey Jake, and the librarian Diana Bellamy as Mrs. Pennington.
Elias calls INTEC representative Mr. Hauser and demands action against the cockroaches, or he will reveal information to the national media. Hauser agrees to spray the island with lethal insecticide at 5 a.m. Elias adds that if residents cannot evacuate, they will turn on the lighthouse beam as a signal not to spray. Hubbard notices that cockroach eggs that survived the rotenone have hatched and are now immune.
Richard finds Millie and Lillian dead. Homer accidentally blows up his house while trying to kill the cockroaches. Richard and Homer confront Hubbard, who reveals that INTEC created a species of cockroach designed to eat other cockroaches and then die after one generation; however, the engineered cockroaches survived and reproduced. To prevent INTEC from spraying the island and thereby making the eggs in the cave immune to the poison, Tarbell and Homer set out to turn the lighthouse beam on.
Elias and Elizabeth are attacked by cockroaches in Elias’s home, including a cat-cockroach hybrid that Homer crushes. It is revealed that the cockroaches can mutate into hybrids reflecting the prey they have eaten. Elias is killed and returns as a human-cockroach hybrid, which Elizabeth kills with a shotgun. Elizabeth, Hubbard, and Tarbell go to the cave to find the nest, while Homer heads to the lighthouse. Hubbard is killed by a queen cockroach-human hybrid, and Elizabeth and Tarbell detonate an explosive inside the cave, incinerating all the cockroaches inside. Homer flips the lighthouse beam on, stopping INTEC from spraying the island.
Yet the film closes on a grim note: a single surviving cockroach crawls away, suggesting the cycle of terror could begin anew.
