As New Mexico State Police Sgt. Ben Peterson and his partner Ed Blackburn patrol the desolate desert surrounding Alamogordo, they unexpectedly encounter a mysterious sight: a catatonic young girl aimlessly wandering while a damaged caravan nearby hints at a darker fate for its occupants. Rushing to the scene, authorities summon an ambulance in hopes of reviving the girl. Initially unresponsive, she suddenly stirs at the sound of an eerie, high-pitched hum emanating from the ground, her eyes snapping open as if roused by an unseen force. However, as the haunting melody fades away, she collapses back into her silent state, unnoticed by the adults around her.
As night falls, the officers stop by a rural general store, only to stumble upon its owner's lifeless body. Peterson leaves Blackburn behind to secure the crime scene while he takes a moment to digest the disturbing sight. Before he can fully grasp the implications of the murder, Blackburn mysteriously disappears, seemingly lured away by an unexplainable force. Days later, FBI Agent Robert Graham arrives to delve into the strange occurrences. In addition, the Department of Agriculture sends in Dr. Harold Medford and his daughter Pat Medford, experts in myrmecology, to assist with the investigation. During their inquiry, Medford exposes the catatonic girl to the potent fumes of formic acid, a substance associated with ants. The overwhelming scent jolts her from her trance, and in sheer panic, she exclaims, > "Them! Them!"
Accompanying Graham and the Medfords back to the site where the girl was first found, Pat inadvertently uncovers a colossal ant foraging in the desert. Following Medford's guidance, Peterson and Graham unleash a hail of bullets at the ant's antennae, successfully blinding and killing it with their submachine guns. Medford proposes that these gigantic insects have mutated due to radiation from the very first atomic bomb test in Alamogordo, explaining their violent behavior. A helicopter search then commences, leading them to the ants' nest. In a frantic attempt to neutralize the threat, cyanide gas bombs are deployed into the nest. Inside, Pat discovers evidence that two queens have hatched and escaped, each ready to establish new colonies.
The authorities initiate a worldwide hunt to track down these mutant creatures, exploring every possibility. They eventually locate one queen and her offspring aboard a freighter bound from Acapulco, but tragically, the crew is found dead. The Navy acts swiftly, sending a fleet to sink the vessel and eliminate any potential menace it might pose.
As Graham and Pat delve further into the investigation, they find themselves in Brownsville, Texas, where a deranged civilian pilot—who had been institutionalized after claiming alien coercion involving giant ants—becomes a primary suspect. Their pursuit continues to Los Angeles, where a hospitalized patient shares a horrifying account of oversized ants lurking in the drainage system near his window. The disheartening discovery of a father's mutilated body starkly emphasizes that not every investigation concludes neatly. In this case, two young boys are missing, leaving the case veiled in mystery. Determined to uncover the truth, Peterson teams up with Graham and the Medfords to scour the drainage system for the lost children. After traversing perilous terrain, he discovers the boys trapped near the ants' colony, only to be ambushed and killed by a monstrous insect. Graham arrives with armed forces in tow, but they soon find themselves confronted by the remaining queen and her army. The Medfords call for eradication, and the ants are exterminated with flamethrowers, as Pat's father reflects on the extensive implications of atomic testing since Trinity, hinting at a broader cosmic context that may have led to this catastrophic predicament.
