Description
New Mexico State Police Sgt. Ben Peterson and Trooper Ed Blackburn discover a little girl wandering the desert, near Alamogordo, in shock and in a catatonic state. They take her to a nearby vacation trailer, located by a spotter aircraft, where they find evidence that the little girl had been there when it was attacked and nearly destroyed. It is later discovered that the trailer was owned by an FBI Special Agent named Ellinson, on vacation with his wife, son, and daughter; the other members of the girl\'s family remain missing. Now in an ambulance, the child briefly reacts to a pulsating high-pitched sound from the desert by sitting up in the stretcher. No one else notices her reaction, and she lies back down when the noise stops.
At a general store owned by \"Gramps\" Johnson, Peterson and Blackburn find him dead and a wall of the store partially torn out. After a quick look-around, Peterson leaves Blackburn behind to secure the crime scene. Blackburn later goes outside to investigate a strange, pulsating sound. Gun shots are fired, the sound grows faster and louder, and Blackburn goes missing. Peterson\'s captain later points out that both Johnson and Blackburn had fired their weapons at their attacker. More puzzling is the coroner\'s report on Johnson\'s brutal death, which includes a huge amount of formic acid found in his body.
The FBI sends Special Agent Robert Graham to New Mexico to investigate because one of the missing persons is an FBI Agent. After a strange footprint is found near the Ellisons\' trailer, the Department of Agriculture sends myrmecologists Dr. Harold Medford (Edmund Gwenn) and his daughter, Dr. Pat Medford, to assist in the investigation. The elder Medford exposes the Ellinson girl to formic acid fumes, which rips her from her catatonia into a state of panic from \"Them!\" Medford\'s suspicions of Camponotus vicinus are validated by her reaction, but he will not reveal his theory prematurely.
At the Ellinson campsite, Pat encounters a giant, eight-foot-long foraging ant. Following instructions from the elder Medford, Peterson and Graham shoot off the ant\'s antennae, blinding it, and kill it using a Thompson submachine gun. Medford reveals his theory: a colony of giant ants, mutated by radiation from the first atomic bomb test near Alamogordo, is responsible for the deaths. General O\'Brien orders an Army helicopter search, and the ants\' nest is found. Cyanide gas bombs are tossed inside, and Graham, Peterson, and Pat descend into the nest to check for survivors. Deep inside, Pat finds evidence that two queen ants have hatched and escaped to establish new colonies.
Peterson, Graham, and the Medfords join a government task force which covertly begins to investigate reports of unusual activity. In one a civilian pilot (Fess Parker) has been committed to a mental hospital after claiming that he was forced down by UFOs shaped like giant ants. Next, the Coast Guard receives a report of a giant queen hatching her brood in the hold of a freighter at sea in the Pacific; giant ants attack the ship\'s crew, and there are few survivors. The freighter is later sunk by U.S. Navy gunfire.
A third report about a large sugar theft at a rail yard leads Peterson, Graham, and Major Kibbee to Los Angeles. An alcoholic (Olin Howland) in a hospital \"drunk tank\" claims he has seen giant ants outside his window. The mutilated body of a father is recovered, but his two young sons are missing. Peterson, Graham, and Kibbee find evidence that they were flying a model airplane in the Los Angeles River drainage channel near the hospital. Martial law is declared in Los Angeles, and troops are assigned to find the ant nest in the vast storm drain system under the city.
Peterson finds the two missing boys alive, trapped by the ants near the nest. He calls for reinforcements and lifts both boys to safety, just before being attacked and grabbed by an ant in its mandibles. Graham arrives with reinforcements and kills the ant, but Peterson dies from his injuries as others swarm to protect the nearby nest. Graham and the soldiers fight off the ants, but a tunnel collapse traps Graham. Several ants charge, but he is able to hold them off with his submachine gun just long enough for troops to break through. The queen and her hatchlings are discovered and quickly destroyed with flamethrowers. Dr. Medford offers a philosophic observation: \"When Man entered the Atomic Age, he opened the door to a new world. What we may eventually find in that new world, nobody can predict.\"
