A young girl is found wandering in shock in the New Mexico desert near White Sands by two state troopers. When they investigate they find that the trailer that she was staying in with her parents and sibling had been destroyed. They also find that kindly old Gramps Johnson who ran the local store was also killed in a mysterious way. A lone print fround in front of the little girl's trailer is sent to the FBI in Washington for identification. Dr. Medford and his daughter Dr. Pat Medford come to New Mexico after identifying the print as belonging to a species of ant that had mutated after the first atomic bombing 9 years before in 1945. They find and destroy the New Mexico nest but not in time to prevent two young queen ants from escaping. One of the ants starts a nest on a military ship at sea. After the ants kill the entire crew the ship is sunk and the invading ants are killed. Another ant makes her way to the drain system of Los Angeles. A father and his two young boys accidently come upon one off the ants. The father is killed and then a hunt is on to see if they are in time to save the two young boys and also destroy the nest before any other ant can escape and destroy the world as we know it. Top notch entertainment!
In the desert of New Mexico, a young girl wandering along a road in shock is found by two state troopers. When she's brought to the police station, she's identified and the police officers decide to investigate the home where she lives with her parents and brother. One side of the house has been smashed down and the place is nearly destroyed. One of the troopers hears a bizarre ringing noise outside and walks around the back of the house to find the source. The man screams, fires a few shots and is silent, attacked and killed by an unseen enemy.
At the police station the young girl meets a scientist who has her smell a pungent substance. She becomes instantly terrified, backing into a corner and screaming the word "THEM!!!"
Title (Note: the movie is black and white but the title is in color - red and blue) and credits run over a desert vista with a large desert tree in the foreground. We see a spotter aircraft fly over the desert. Johnny (an uncredited John Close) calls the car below to report something. A little girl is seen wandering in the desert. Johnny circles until the police car arrives. The girl (Sandy Descher) is dressed in a bathrobe and carrying a doll. Sgt. Ben Peterson (James Whitmore) exits the car to retrieve the child. She is in shock and unresponsive to commands. Trooper Ed Blackburn (Chris Drake) takes a call from the airplane pilot. Johnny has spotted a trailer about three miles down the road. Ben and Ed take the girl and drive down the road to investigate the trailer. They discover the trailer has sustained structural damage but there is no evidence of fire or an explosion. No money has been taken and blood stains are estimated to be less than twelve hours old. Ben finds a piece of cloth and plastic. Ed finds some sugar cubes. A mysterious print in the sand piques their curiosity. Ben matches the cloth from the little girl's bathrobe and the plastic piece fits the missing piece of her doll's head. An ambulance is called and a police team goes over the crime scene. The ambulance attendant (an uncredited William Schallert) assures Ben he'll take good care of the child. They hear a weird chirping noise like crickets, but do not notice the Ellinson girl respond to the sound. They attribute the sound to the wind. Ben asks Cliff (an uncredited Cliff Ferre), who is finishing the plaster moulage on the strange print, what he thinks it might be. The ambulance departs, so Ben and Ed decide to visit the local store run by Gramps Johnson (an uncredited Mathew McCue). They want to know if he knows anything.
It is early evening when Ben and Ed arrive at the store. The wind is howling and the store is a shambles. They investigate and discover there is no sign of robbery, but a twisted shotgun indicates Gramps fought off his attackers. They find his dead body in a storage area below the store. Outside the two officers hear a noise. They notice the wall has been pulled away, just like at the trailer. A large barrel of sugar has been turned over. Ben exits the store and leaves Ed to remain until the investigation team arrives. Ed hears the weird chirping sound and goes outside to investigate. We only hear the confrontation -- six shots from Ed's service revolver and a scream.
At police headquarters the clues from both scenes are spread out on the table in the office of Capt. Fred Edwards (Don Shelton). They are stumped to make any sense from the clues, including the strange plaster print. They do get a match on fingerprints from the trailer. The Ellinson family was on an extended vacation and the father was an FBI agent. Robert Graham (James Arness), an FBI agent from the office in Alamogordo, joins the investigation. Bob Graham is also stumped. Bob suggests sending the print to Washington for identification. The coroner, Doc. Putnam (an uncredited Joseph Forte) arrives and provides five causes of death. He tells them, "Johnson could have died in one of five ways; His neck and back were broken, his chest was crushed, his skull was fractured...and here's one for Sherlock Holmes: There was enough formic acid in him to kill twenty men."
Bob Graham receives a telegram from the Bureau telling him to meet Doctors Medford at the airport and extend all cooperation. The FBI was unable to identify the print and sent it to the Department of Agriculture. A military transport, an old North American B-25 Mitchell, arrives and the first out is Dr. Harold Medford (Edmund Gwenn). He is short, plump, and slightly distracted in his own thoughts. Next off the plane is Dr. Medford's daughter, Patricia (Joan Weldon). She is tall, brunette, and very attractive and prefers to be called Pat. Bob Graham immediately takes a shine to this scientist. The four return to police headquarters. The Medfords review the clues and read all the reports. Dr. Medford asks where exactly the first atomic bomb in 1945 was exploded. He is told in the same general area. Dr. Medford thinks out loud, "1945. That's nine years ago. Yes, genetically its certainly possible." Bob wants to know what they suspect, but Dr. Medford refuses to explain until he and his daughter are certain of their theory. Dr. Medford wants to stop at a drug store and visit the Ellinson girl at the hospital, then revisit the location where the print was found.
At the hospital, Medford examines the girl and consults with the psychiatrist (an uncredited Ann Doran). A small glass filled with formic acid causes the girl to react. She runs and tries to hide while screaming, "Them! Them!" The four exit the hospital room and visit the trailer site. The wind is howling and the blowing sand and dust make it hard to see and hear each other. Pat remarks that, "Rather slim pickings for food, Dad. They'd turn carnivorous for lack of a habitual diet." Medford agrees, then walks away. Bob wants to know what they are talking about and gets rather insistent. Pat refuses to cooperate, but explains to Bob that her father is an expert myrmecologist, and will tell him when they are sure. Medford discovers a print, and measuring it determines the creature would be over eight feet long. Pat wanders off to find more prints. She discovers one, then she hears the weird chirping sound. It intensifies and the source reveals itself as a giant ant. She screams and backs away. Bob comes to her rescue firing his revolver at the creature, but with little effect. Medford yells they should aim for the antenne, as they are helpless without them. Ben retrieves a machine gun from the police car and fires enough bullets into the creature to kill it. They all approach to get a closer look at the insect. When asked what it is, Medford replies, "Species appears to be Camponotus Vicinus. One of the family Formicidae. An Ant." Medford speculates that radiation caused this mutation.
General O'Brien (Onslow Stevens), Ben and Medford are aboard a helicopter searching the desert for the nest of the giant ants. Aboard a second helicopter is Bob, Pat, and Major Kibbee (Sean McClory). They are also searching for the nest in another sector. Pat admits she is worried about her father. Pat finds what she is looking for -- a cone shaped mound with a large opening on top. They circle the mound and see an ant remove a human rib cage and throw it into a pile. Human skulls, a police belt with bullets and other bones are littered at the base of the mound. Pat proclaims, "You've just found your missing persons."
In an FBI office, General O'Brien confronts Dr. Medford about his inconsistency. Medford insists secrecy is paramount and that time is the most important thing. O'Brien is anxious to bomb the nest and be done with the affair. Medford explains that the ants won't be in the nest at night. He proposes to attack them during the day with poison gas, then examine the nest to verify all are dead. During the hottest part of the next day, O'Brien and Ben fire a bazooka with phosphorus shells onto the mound to keep the area hot and the ants below. Bob Graham and Maj. Kibbee also lace the mound with incendiary shells. The Medfords watch and wait. Bob and Ben approach the opening with cyanide gas grenades. A worker ant is seen trying to escape the nest and is held off with a machine gun. The two men bombard the nest with the gas grenades.
Bob and Ben prepare to enter the nest. Pat approaches dressed for a subterranean exploration. She tells the men she will accompany them. Bob objects, but Pat explains, "Someone with scientific knowledge has to go. My father is physically unable to do it. That leaves me." Bob reluctantly agrees and the three descend into the nest. At first, all they find are dead ants. Further along and down, they are surprised when a wall begins to collapse and they are confronted by some live ants. They are quickly dispatched with Ben's flamethrower. In the queen's chamber, Pat is disappointed to discover they are too late. Without explaining, she takes a few photographs then orders the contents burned. Ben torches all the eggs and dead adult worker ants.
Dr. Medford explains that the empty egg cases contained queen ants, which will set up new nests. The team goes to Washington, D.C. to brief a panel of military and government personnel. Medford shows them a film about ants to give them some idea of what they face in combating the insects. Medford punctuates his presentation with a warning if they cannot be stopped, "Man, as the dominant species of life on Earth will probably be extinct within a year." A collection center for information is established and all reports of strange phenomena are funneled through same. An Air Force Sergeant (an uncredited Leonard Nimoy) pulls a dispatch off a teletype machine and hands it to a woman to classify. The note refers to a pilot, Alan Crotty, crashing his airplane after witnessing UFOs shaped like ants. He confirms that everything in the building is top secret. Bob and Pat review the dispatch and fly to Brownsville, Texas with Maj. Kibbee to investigate the Crotty report.
Alan Crotty (Fess Parker) reluctantly tells his story to what he perceives as an unbelieving audience. Bob, Pat and Kibbee listen and ask a few questions. The three exit Crotty's room and speak with the doctor. Bob tells the doctor to keep Crotty confined. Back in Washington, the Medfords and Bob Graham review what they know so far. Medford takes a call and rushes to the communications room. O'Brien is there and directs Medford to an incoming message. The U.S.S. Viking reports a nest of ants hatched aboard their ship at sea. Only two survivors are rescued by the Navy. The Viking was ordered sunk by Naval gunfire. The Medfords conclude a queen flew aboard and hatched a nest while the ship was anchored in Acapulco, Mexico for the three days and four nights it was there. An official at the meeting (an uncredited Willis Bouchey) is convinced the public should be notified of the danger, but the consensus is to keep things secret to avoid panic. Pat reports that Kibbee, Ben and Bob are in Los Angeles investigating a lead -- the theft of forty tons of sugar.
It is Sunday morning and Ben and Bob talk to the rail yard detective (an uncredited Dick Wessel). He reports that the rail car was broken into on Friday night and the yard watchman is being held as a suspect. At police headquarters Bob questions the yard watchman (an uncredited Dub Taylor). Ben was questioning Mrs. Lodge (Mary Alan Hokanson) about the death of her husband. Bob and Ben go to the morgue to see the body and talk to the coroner (an uncredited Russell Gaige). The coroner explains, "I don't think this happened in a machine." The missing arm and laceration on the chest are curious. He attributes death to shock and loss of blood. Lodge was with his two children, but he was found alone in his car without any trace of the arm. Bob questions Mrs. Lodge but she tells him she has no clue where her husband took the children that morning. Ben interrupts the interrogation to question patrolmen Ryan (an uncredited John Beradino) and Sutton (an uncredited Robert Berger). They found Thomas Lodge. They show Bob and Ben on the map of the city where they found the body. Bob and Ben interview the four people cited by the officers three drunks and a speeding ticket. A blonde floozy (an uncredited Mary Lou Holloway) is questioned and released. Two of the three drunks are returned to the tank to sleep it off. Ryan explains that the third drunk is at the hospital in the alcoholic ward.
The ward doctor (an uncredited Grandon Rhodes) escorts Kibbee, Bob and Ben to see Jensen (Olin Howlin). Jensen tells them that he saw a little airplane and large ants down in the open storm drain channel. Bob comments that the openings look like they may be worth investigating. Jensen's loud outburst disturbs the alcoholic in the next bed (an uncredited Harry Wilson) who pleads, "Please, my nerves!" Ryan drives Bob, Ben and Kibbee down onto the river bed. They search the area and find a model airplane. Kibbee finds an ant print, which confirms the drunk's story.
A news conference is called on Sunday evening. General O'Brien makes the announcement, "By direction of the President of the United States, in full agreement with the Governor of the State of California, and the Mayor of Los Angeles. The city of Los Angeles is, in the interest of public safety, hereby proclaimed to be under martial law. Curfew is at 1800 hours. Any persons on the street or outside their quarters after 6:00 p.m. tonight will be subject to arrest by the military police." O'Brien explains what prompted such drastic action. He explains that giant ants were discovered in New Mexico and a nest has been established in the storm drains under the city of Los Angeles. The military begins to stage for the attack. A field headquarters is established outside the storm drain opening where the model airplane and print was discovered. Search units in Jeeps proceed into the storm drains. About a mile in, Ben hears something and requests all engines be stopped. He investigates a pipe that leads to a section still under construction. He finds Mike Lodge (an uncredited Richard Bellis) and his brother Jerry Lodge (an uncredited Robert Scott Correll) trapped and about to be attacked by the ants. Ben kills the ants with his flamethrower. He rescues the boys and sends them down a pipe back to the safety of his waiting driver, Smitty (an uncredited Joel Smith). Unfortunately his own escape is thwarted. An ant grabs Ben with its mandibles and crushes him to death. All units are sent to Drain 267 to engage the enemy. O'Brien and Dr. Medford drive to the location. Bob, and a unit of soldiers fight their way to the nest area, killing many ants with gunfire. O'Brien and Medford arrive and order a cease fire. A section of tunnel collapses trapping Bob inside with the ants. Soldiers dig away the rubble and reach Bob before he is killed by the ants guarding the nest of newly hatched queens. All the worker ants protecting the nest are killed. Medford is called and examines the nest. He says, "This is the egg chamber. I'm certain no new queens have escaped from this nest. The job will be done when these are destroyed." O'Brien orders, "Okay, burn them out." Flamethrowers destroy the winged queen and consort ants. Dr. Medford waxes philosophical, "When man entered the atomic age, he opened a door into a new world. What we'll eventually find in that new world, nobody can predict." We close with a slow zoom in shot of the ants on fire, some still moving.