Stressed by a faltering marriage and alcohol problems, Dr. Benjamin Cahill freezes under pressure in the emergency room. His superior orders him to take a break, and he travels to Orr island, a quiet fishing community off the coast of Maine, to unwind and regroup at the house he and his ex once shared.
There, at the request of Sheriff Hobbs, Cahill begins examining a string of animal and human corpses that show strange internal and external injuries. He notices that a red cockroach population that has recently infested the island—and the house—features pincers that are oddly developed for its species. His growing curiosity leads him to consult university entomology sources, confirming that this African cockroach is something of an anomaly, and the infestation only intensifies Cahill’s sense of unease.
Tension with the locals escalates quickly. A man named Jack Wald breaks into Cahill’s home and attempts to trash it, only to be overwhelmed and killed by the cockroaches. The fear and distrust deepen, and Cahill is allowed to perform an autopsy on Jack, where he discovers that these cockroach-like insects burrow into people and nest inside them, liquifying internal organs before bursting out of the host.
A mob forms and attacks Cahill, imprisoning him and his girlfriend, Nell Bartle. The collapse of order during this rising crisis makes Cahill’s explanations seem increasingly plausible to some and increasingly suspect to others.
During the turmoil, Eamon Wald assaults Sheriff Hobbs as he tries to stop the break-in, and Hobbs is locked in the basement and killed by the insects. His body is later found by Guy Barter, who had tried to free him, further fueling the town’s fear and anger.
When Cahill and Nell are eventually released, they begin evacuating locals, starting with Nell’s mother, Mrs. Bartle, and the children in her school class, though one pupil remains missing. They soon uncover that the cockroaches are nesting in a barn, a discovery that reinforces the escalating sense of danger around every corner.
A swarm of mutant insects is seen overhead, and the reluctant locals scramble to flee. Many perish, including the once-timid resistance of Eamon and his crew. Cahill, Nell, the rescuer Guy, and Henry S. Crump take to the water in a boat and push toward the mainland, where they are attacked again. They overturn the boat and ignite a fuel pump, incinerating the bugs just as the military is called in to handle the crisis. Yet the final shot reveals that one of the creatures has made it to the mainland, leaving the ending shrouded in ambiguity.
