Sergeant Ben Peterson (James Whitmore) and trooper Ed Blackburn (Christian Drake) of the New Mexico State Police discover a little girl wandering in the desert near Alamogordo. She is in shock and nearly catatonic. They take her to a nearby trailer and discover that she was there when it was destroyed. Put in an ambulance, she hears a high-pitched pulsating sound coming from the desert and sits right up, then slips back into her catatonic state. At Gramps Johnson’s store, they find the old man dead and the wall ripped off the store. Blackburn investigates a pulsating sound, fires his gun, and disappears. Johnson’s body contains high levels of formic acid.

The FBI sends Special Agent Robert Graham (James Arness) to investigate because one of the missing persons is an FBI agent. A strange impression in the sand, not really like a footprint, is found near the wrecked trailer. The Department of Agriculture sends myrmecologists Doctor Harold Medford (Edmund Gwenn) and his daughter Doctor Pat Medford (Joan Weldon). Medford exposes the girl to formic acid fumes. She screams in a panic and shouts, “Them!” He speaks of Camponotus Vicinus but will not explain what that means.

However, Pat encounters a giant, eight-foot-long ant. The cops shoot off the ant’s antennae as instructed and then manage to kill it with a Tommy gun. Doctor Medford theorizes that a colony of ants, mutated by the first atomic bomb test at Alamogordo, have grown to enormous size. A helicopter search discovers their nest and cyanide gas bombs are tossed inside. Graham, Peterson, and Pat descend into the maze of tunnels and find evidence that two queens have hatched and escaped to start new colonies.

Peterson, Graham, and the Medfords join a government task force investigating strange happenings in the area. A civilian pilot, Alan Crotty (Fess Parker) is in the mental hospital, claiming that he was forced down by UFOs that looked like giant ants. The Coast Guard reports a giant Queen Ant hatching her brood in the hold of a freighter at sea. There is a report of a huge sugar theft at a railyard in Los Angeles. A father and his sons flying a model plane in the dry Los Angeles River are dead or missing, and Martial Law is declared.

Troops enter the huge storm-drain system under the city and Peterson finds the missing boys alive. As they are being lifted to safety, he is grabbed by the mandibles of an enormous ant, and Peterson dies as the ants swarm. The tunnels collapse and Graham is trapped. He holds off the ants with a machine-gun and the troops break through. The queen and her hatchlings are found and destroyed with flamethrowers. Doctor Medford muses about the new world that the atomic age has opened.

The film was directed by Gordon Douglas and produced by David Weisbart, based on a story by George Worthing Yates and a screenplay by Ted Sherdeman and Russell Hughes. It is supposedly the first of the Fifties big bug pictures and probably the best, mostly because of the acting. If the characters believe the nonsense going on around them, we do too. Walt Disney watched the film to see if he should hire James Arness to play Davy Crockett but was impressed by Fess Parker’s small role. John Wayne saw the movie and recommended James Arness for the Gunsmoke series. Leonard Nimoy plays an Army staff sergeant in the communications room.

The film was originally supposed to be made in 3-D and color, and then in black-and-white widescreen, but it was neither. You can still see shots in the movie designed for 3-D, like flamethrowers aimed at the camera, and the titles are in shocking color. Warner Brothers was hoping for the same success as their Beast from 20,000 Fathoms of the previous year. James Whitmore had to wear lifts in his shoes to stay in the same frame as James Arness. The ants were operated by hidden technicians inside. Their sound was mixed tree-frogs and birds. It was pretty well received by critics and has since become a classic, up there with The Thing and Body Snatchers.

Van Morrison named his rock-group Them after the movie and a scene from the film appeared in Ant-Man and the Wasp. The flame-throwers were loaned by the US Army, as well as the soldiers who had used them in battle. There were always no more than three giant ants on the screen at one time because that’s all they had. The film lost the Oscar for Special Effects to 20,000 Leagues Under the Sea. It still has 93% on Rotten Tomatoes. The scene of torching the queen ant’s eggs must surely have influenced the similar scene in Aliens.