One morning, in Jul 1951, people around the world are astonished by the appearance of an unidentified, fast-moving object circling the globe. The object, a large, metallic saucer, lands on the Mall in Washington, D.C., where a crowd quickly gathers. Soldiers surround the saucer, and tension grows until a helmeted man emerges, telling the crowd that he has come in peace. A nervous soldier shoots the man when he brandishes a small object, and people run in terror as an enormous robot lumbers out of the saucer. The robot, Gort, emits a mysterious ray that melts the soldiers' weapons until the wounded alien, Klaatu, orders him to desist and informs the soldiers that the object was a gift for their president. Klaatu is then rushed to Walter Reed Hospital, where presidential secretary Harley learns that he has traveled over 250,000,000 miles in five months. Klaatu reveals that he must talk to all of Earth's leaders, but Harley explains that due to the unstable political climate, not all of the leaders will consent to meet with one another. Klaatu insists that his mission is too important to be derailed by petty squabbles, and warns Harley that the future of the planet is at stake. The next day, doctors find that Klaatu has recovered from his wound, after which Harley returns with news that although the president has invited the world's leaders to meet, many have refused unless they can host the gathering. Bemused by their childish jealousies, Klaatu decides to learn more about the species he is visiting, and escapes from the hospital. Radio and television broadcasts announce Klaatu's escape, but no pictures of him are available. Using the name Carpenter, Klaatu obtains a room in the boardinghouse of Mrs. Crockett, which is also inhabited by Mr. and Mrs. George Barley, Mr. Krull and an attractive widow, Helen Benson, and her young son Bobby. The next day, Helen and her beau, Tom Stevens, go on a picnic, and Klaatu babysits Bobby, who takes him on a tour of the city. Klaatu is impressed with the Lincoln Memorial, and when he asks Bobby who the greatest man alive is, the boy replies that famed scientist Jacob Barnhardt is the one Klaatu seeks. After taking Bobby to visit the saucer, Klaatu goes with him to Barnhardt's house, where he easily solves a math problem begun by the absent Barnhardt. That night, Helen watches as a government agent picks up Klaatu at the boardinghouse. Klaatu is taken to see Barnhardt, who is thrilled to learn that he is the alien. Klaatu explains that because Earth's people are reaching a technological level at which they could be a danger to other planets, he has been sent to warn them of the consequences of their violence. Barnhardt speculates that no one will take his message seriously unless he can prove his superior capabilities, so Klaatu promises to arrange a demonstration in two days. The next evening, Tom belittles Klaatu's involvement with Helen and Bobby, although Bobby states that Klaatu is his best friend. After Helen and Tom leave, Bobby follows Klaatu as he goes to the saucer, where he signals to Gort and obtains entrance to the vessel. Inside, Klaatu arranges his demonstration, while Bobby runs home. When Helen and Tom return, Bobby tells them of his adventure, but they dismiss it as a bad dream. Tom grows worried though, when he snoops in Klaatu's room and finds one of the diamonds that Klaatu trades for currency. The following day, Klaatu gets Helen from her office, and when they enter the building's elevator, the power suddenly dies, and Klaatu reveals his identity to Helen. As he discusses his mission with her, people around the world are confused by the loss of all electricity, except to critical locations such as hospitals. Barnhardt is gleeful about the nonviolent demonstration, which he believes will ensure full attendance for his meeting that night of scientists and social leaders. Half an hour later, when the power is restored, Helen is convinced of Klaatu's sincerity, and is determined to help him. She is concerned about Tom, however, as he has taken the diamond to a jeweler for appraisal. Helen rushes to Tom's office, where she is dismayed to learn that he is more interested in the fame and fortune that will result from betraying Klaatu than in the reason for his visit. While Tom alerts the Pentagon, Helen storms out and whisks Klaatu away from the boardinghouse just before the military arrives. During their ride to Barnhardt's meeting, Klaatu tells Helen that if anything happens to him, she is to find Gort immediately and tell him "Klaatu Barada Nikto" to prevent him from destroying the Earth. Before they can reach the ship, however, Helen and Klaatu are stopped by soldiers, and Klaatu is fatally shot. Helen escapes and goes to Gort, who responds to her repetition of Klaatu's orders by carrying her inside the saucer, then stealing Klaatu's body and returning it to the ship. There, Gort revives Klaatu, although Klaatu tells Helen that his re-animation will last for an uncertain duration. Klaatu then appears before Barnhardt's colleagues and warns them that because the universe grows smaller every day, threats of aggression cannot be tolerated. Klaatu assures them that they will not lose any freedoms, but that they will be under the watchful eye of a robot such as Gort, who was invented by an organization of planets determined to stamp out violence. Each planet is guarded by a robot that impartially acts against aggression, and the inhabitants live in peace, Klaatu states, with their energies turned toward higher goals. Klaatu informs the crowd, "Your choice is simple: join us and live in peace, or pursue your present course and face obliteration." With a final smile for Helen, Klaatu then leaves with Gort.