When a flying saucer lands in Washington, D.C., the Army quickly surrounds it. A humanoid (Michael Rennie) emerges from the spacecraft and announces that he comes “in peace and with good will”. When he unexpectedly removes and opens a small device, he is shot and wounded by a nervous soldier. A tall robot emerges from the saucer and quickly disintegrates the Army’s weapons. The alien orders the robot, Gort, to desist. He explains that the now-broken device was a gift for the president of the United States that would have enabled him “to study life on the other planets”.
The alien, Klaatu, is taken to Walter Reed Hospital. After surgery, he uses a salve to quickly heal his wound. Meanwhile, the Army tries but is unable to enter the saucer; Gort stands outside, silent and unmoving.
Klaatu tells the President’s secretary, Mr. Harley (Frank Conroy), that he has a message that must be delivered to all of the world’s leaders simultaneously. Harley tells him that such a meeting in the current political climate is impossible. Klaatu suggests that he be allowed to go among humans to better understand their “unreasoning suspicions and attitudes”. Harley rejects the proposal, and Klaatu remains under guard.
Klaatu escapes and lodges at a boarding house as “Mr. Carpenter”, the name (“Maj. Carpenter”) on the dry cleaner’s tag on a suit he acquired. Among the residents are young widow Helen Benson (Patricia Neal) and her son Bobby (Billy Gray). The following morning, Klaatu listens to the boarders speculate about the alien’s motivations.
While Helen and her boyfriend Tom Stevens (Hugh Marlowe) are not at home, Klaatu is a babysitter for Bobby. The boy takes Klaatu on a tour of the city, including a visit to his father’s grave in Arlington National Cemetery; Klaatu learns that most of the deceased are former military servicemen killed in wars. They visit the Lincoln Memorial, and at the heavily guarded spaceship Klaatu asks Bobby who the greatest living person is; Bobby suggests Professor Barnhardt (Sam Jaffe). Bobby takes Klaatu to Barnhardt’s home, but the professor is not home. Klaatu enters the professor’s locked study from outside and adds a mathematical equation on a blackboard to assist Barnhardt in solving a celestial mechanics problem; he leaves his contact information with the suspicious housekeeper.
During the evening, a government agent accompanies Klaatu to Barnhardt. Klaatu explains that the people of other planets are concerned now that humanity has developed rockets and a rudimentary form of atomic power. Klaatu declares that if his message is ignored, “Earth will be eliminated”. Barnhardt asks Klaatu if such power really exists. Klaatu assures him that it does. Barnhardt agrees to gather scientists from around the world at the saucer; he then suggests that Klaatu give a harmless demonstration of his power. Klaatu returns to his spaceship that night, unaware that Bobby has followed him. Bobby sees Gort render two soldiers unconscious and Klaatu enter the saucer.
Bobby tells Helen and Tom what he saw, but they do not believe him until Tom takes a diamond he found in Klaatu’s room to a jeweler and learns it is “unlike any other on Earth”. Klaatu finds Helen at her workplace, and they take an empty service elevator, which stops precisely at noon. He has neutralized all electricity everywhere for 30 minutes, except for such essential services as hospitals and airplanes in flight. Klaatu reveals his true identity to Helen, asks for her help, and explains his mission.
After Tom informs the authorities of his suspicions, Helen breaks up with him. She and Klaatu decide to visit Barnhardt’s home. On the way there, he tells her that if anything happens to him, she must say to Gort, “Klaatu barada nikto”. Their taxi is spotted and hemmed in. Klaatu makes a break for it and is shot dead. Helen quickly heads to the saucer. After killing two soldiers standing watch, Gort advances on her, and Helen utters Klaatu’s words. Gort carries her into the spaceship and leaves to retrieve Klaatu’s body. Back in the saucer, Gort revives Klaatu, and he explains to Helen that his revival is only temporary.
Klaatu addresses Barnhardt’s assembled scientists, informing them that he represents an interplanetary organization that created a police force of invincible robots like Gort. “In matters of aggression, we have given them absolute power over us”. Klaatu concludes, “Your choice is simple: join us and live in peace, or pursue your present course and face obliteration. We shall be waiting for your answer”. Klaatu and Gort re-enter the saucer and depart.