The film begins with New Mexico State Police Sergeant Ben Peterson (James Whitmore) discovering a little girl wandering the desert near Alamogordo, New Mexico, mute and in a state of shock. They track her back to a trailer owned by an FBI agent named Ellinson, who was on vacation in the area with his wife and daughter. The side of the trailer is found to have been ripped open from the outside, the sugar bowl is spilled across the table and the parents are missing and presumed dead. The girl briefly responds when strange sounds echo out of the desert wind, but the troopers miss this moment.
More mysterious deaths and disappearances occur in the area. A general store owner named Gramps Johnson is found dead, his store literally torn apart. With all the money left in the register but all of the store's sacks of sugar missing and Gramps' empty rifle bent on the floor, the cops think that there is a maniac killer on the loose. But, as Peterson's boss points out (after Peterson's patrol partner Ed Blackburn, played by Chris Drake, was killed too, while strange ullulating echos mingle with his gunshots and cries), Gramps' 30-30 was emptied and "Ed Blackburn was a crack shot. He could hit anything he could see. So unless your killer is armored like a battleship, there's no maniac in this case." It's up to the coroner to deliver the verdict that "Gramps Johnson could have died in any one of five ways: his neck and back were broken, his skull was fractured, his chest was crushed, and here's one for Sherlock Holmes: there was enough formic acid in his body to kill twenty men."
The FBI sends in Ellinson's fellow agent Robert Graham (James Arness) to assist. A single strange track as big as a mountain lion's is found in the desert near the trailer and a plaster cast of it is made and sent to Washington, DC. When the FBI is unable to identify the footprint, it attracts the attention of Doctors Harold (Edmund Gwenn) and Pat Medford (Joan Weldon), a father/daughter team of entomologists from the Department of Agriculture.
The elder Doctor Medford arrives on the scene with a theory, but will not disclose it until he tries an experiment on the Ellinson girl, having her smell the contents of a vial of formic acid, which frees her from her state of near-catatonic withdrawal, screaming "Them! Them!" Returning to the destroyed trailer with Peterson, Graham, and his daughter, Medford has his theory dramatically given its final proof when the group encounters a patrol of foraging ants, mutated by atomic radiation to the size of automobiles. The alarmed ants produce loud, distinctive stridulating calls that become the iconic signature of the beasts. The lawmen kill one of the ants with a Thompson submachine gun after finding that their revolvers have little effect. They aimed for the antennae on Medford's advice that they were helpless without them.
A company of the US Air Force is brought in, led by General O'Brien (Stevens), which locates the ants' nest and exterminates the inhabitants with poison gas. The younger Dr. Medford, who accompanies Peterson and Graham into the nest, finds evidence that two young queens have hatched and flown away to establish new colonies. Trying to avoid a general panic, the government covertly monitors and investigates any reports of unusual activities as sightings of "flying saucers". One of the queens ends up in the hold of an ocean-going freighter loaded with sugar, which is then overrun by the ants and subsequently sunk by a US Navy cruiser. From the rantings of an alcoholic, and an investigation into the death of a father protecting his two young, now missing, sons from an apparent ant attack, the other queen is finally tracked to the Los Angeles storm drain system, forcing the Army to openly declare martial law and launch a major assault.
During the assault, Peterson finds the two missing boys, named Mike and Jerry, alive, trapped by the ants in a side tunnel, which is also the entrance to the nest. Peterson calls in for backup, but instead of waiting for it, he bravely goes in alone, heroically rescuing the two boys and killing numerous ants with his flamethrower. Peterson leads the two boys back to the pipe through which he came, intending that they all crawl back through it to safety. After hoisting up the first boy, Jerry, however, another ant appears from behind, and thinking quickly and selflessly, Peterson saves the second boy, Mike, but after lifting the boy into the pipe, Peterson is left without time to save himself. As he tries to climb up into the pipe at the last minute, the ant grabs Peterson in its mandibles and crushes him at the waist, the man crying out in agony all the while.
Graham arrives to the scene quickly with the reinforcments, and kills the ant attacking Peterson. He rushes over to Peterson's side just in time to hear Peterson's last words confirming that the boys made it to safety, before Peterson dies in his arms. Graham respectfully orders Peterson's body to remain undisturbed, and then returns to the battle, nearly getting killed himself when a cave-in temporarily seals him off from the rest of the men as they march towards the egg chamber; several ants charge him, but Graham is able to hold them off long enough for the other troops to tunnel through the debris and come to his rescue. The nest's queen and egg chamber are then destroyed with flamethrowers after a short but fierce battle, but the senior Dr. Medford issues a grim warning that the atomic genie has been let out of the bottle, and further horrors may await mankind.
Plot review, Wikipaedia