Them!

Two officers find a catatonic girl wandering the desert near Alamogordo, along with the ravaged travel trailer she came from. They call for an ambulance and begin processing the crime scene. The discovery hints at a larger, more dangerous mystery at work.

The investigators head to a nearby rural general store, which has been destroyed in the same manner as the trailer. The store owner's body is found in the cellar, mutilated, and clearly not a robbery. No money is missing from either scene, suggesting something beyond ordinary crime.

Back at the trailer site, Blackburn hears a strange pulsating sound from outside and investigates. He fires at something unseen and experiences a painful, violent shock, screaming in agony. The incident implies a deadly, otherworldly threat at the crime scene.

Agent Robert Graham is called in from the local FBI office to assist Peterson. He forwards a footprint cast to Washington, D.C., and learns that Drs. Harold and Pat Medford, a father-daughter myrmecology team, are being sent by the Department of Agriculture. Their expertise will help identify the insect clue.

Harold Medford exposes the girl to the scent of formic acid, a chemical characteristic of ants. The odor jolts her from catatonia, but she screams 'Them! Them!' in panic. The scientists begin to connect the girl’s condition to an ant-related threat.

Pat Medford is attacked by a giant foraging ant during a field search. Harold directs Peterson and Graham to shoot off the ant's antennae to blind it, then Peterson finishes the creature with a Thompson submachine gun. The encounter demonstrates the ants' dangerous capabilities.

Harold theorizes the ants were mutated by radiation from the first atomic bomb test near Alamogordo and have become carnivorous due to desert scarcity. A helicopter discovers a giant nest surrounded by human remains. Cyanide gas bombs are dropped and Graham, Peterson, and Pat enter to explore the nest.

At the lowest level, Pat finds evidence that two queen ants have hatched and flown away, suggesting the potential for further outbreaks. The team realizes the danger of rapid spread if the queens establish new colonies. Harold warns that humanity could be displaced if the ants proliferate.

The search area broadens as investigators pursue leads across states. Graham and Pat travel to Brownsville, Texas, to interrogate a ranch foreman who claims he was forced down by UFOs shaped like giant ants. The information expands the scope and urgency of the investigation.

A queen ant boards a freighter in Acapulco, and its offspring emerge in the Pacific. The U.S. Navy responds but only finds two sailors alive; the freighter is ordered sunk to prevent spread. The pursuit of the ants reaches transoceanic scales.

A shipment of stolen sugar leads investigators to a Los Angeles hospital, where a chronic alcoholic claims to have seen giant ants in the city’s storm drains. They trace the sightings to the drain network and connect them to the missing queen and two missing boys.

Martial law is declared in Los Angeles and a curfew is imposed as the armed forces and scientists search the 700 miles of tunnels beneath the city. The team locates the remaining queen and confirms no other queens have escaped. The scale of the threat becomes starkly clear.

Peterson finds the two missing boys trapped by ants and calls for reinforcements. He is killed during the rescue attempt, but Graham arrives with backup and the queen along with her hatchlings is located. The nest is destroyed in a final push to end the invasion.

The Medfords confirm no further queens remain and the nest is destroyed, marking a grim victory. Graham wonders what havoc past nuclear explosions might unleash, and Harold notes that humanity has entered a new world with an unpredictable future.