When a flying saucer lands in Washington, D.C., the Army quickly surrounds the spaceship. A humanoid (Michael Rennie) emerges, announcing that he has come in peace. As he advances, he unexpectedly opens a small device and is shot by a nervous soldier. A tall robot emerges from the saucer and quickly disintegrates the soldiers' weapons with an energy ray. The alien orders the robot, Gort, to stop. He explains that the broken device was a gift for the President, which would have enabled him "to study life on the other planets".
The alien is taken to Walter Reed Hospital, where he reveals his name: Klaatu. After surgery, Klaatu uses a salve to quickly heal himself. Meanwhile, the military is unable to enter his 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 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 leaves Klaatu under guard.
Klaatu escapes and lodges at a boarding house as "Mr. Carpenter", the name on the dry cleaner's tag on the suit he "borrowed". Among the residents are young widow Helen Benson (Patricia Neal) and her son Bobby (Billy Gray). The next morning, Klaatu listens to the boarders speculate about why the alien has come.
While Helen and her boyfriend Tom Stephens (Hugh Marlowe) go out, Klaatu babysits 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 those buried there were killed in wars. The two visit the Lincoln Memorial, then the heavily guarded spaceship. Klaatu asks Bobby who is the greatest living person; Bobby suggests Professor Barnhardt (Sam Jaffe), who lives in the capital. Bobby takes Klaatu to Barnhardt's home, but the professor is absent. Klaatu adds a mathematical equation to a problem on Barnhardt's blackboard and leaves his contact information with the suspicious housekeeper.
That evening, a government agent takes Klaatu to Barnhardt. Klaatu explains that the people of the other planets have safety concerns 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 agrees to gather scientists from around the world at the spaceship; he then suggests that Klaatu give a harmless demonstration of his power. Klaatu returns to his ship that night, unaware that Bobby has followed him. Bobby sees Gort knock out two sentries and "Mr. Carpenter" enter the spaceship.
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 abruptly stops precisely at noon. Klaatu reveals his identity and his mission, then asks for her help. He has neutralized all electricity everywhere, except for such things as hospitals and aircraft in flight.
Exactly 30 minutes later, the blackout ends. When Tom informs the military of his suspicions, Helen breaks up with him. Helen and Klaatu take a taxi to Barnhardt's home. En route, he tells her that should anything happen to him, she must tell Gort "Klaatu barada nikto". Klaatu's taxi is spotted and hemmed in; he makes a break for it and is gunned down. Helen quickly heads to the saucer. Gort disintegrates both sentries and advances on her. When Helen utters Klaatu's three words, the robot carries her into the saucer, then leaves to retrieve Klaatu's body. Later, Gort revives Klaatu. Klaatu explains to Helen that his revival is only temporary, that the power of life and death is "reserved for the Almighty Spirit".
Klaatu and Helen emerge from the spaceship and tells Barnhardt's assembled scientists that the people of Earth can join the other planets in peace, but should they threaten to extend their violence into space, "this Earth of yours will be reduced to a burned-out cinder." Klaatu and Gort then fly away.
Output: What does Klaatu tell Professor Barnhardt will happen if his message is ignored?