Regardless of whether or not this film was ever acknowledged as the source of numerous UFO contactees' messages from outer space, it seems likely that at least on the subconscious level, the stately, silver-suited figure of Klaatu (Michael Rennie) and his warning to earthlings to cease their aggressive behavior and live in peace was echoed in countless sermons from alleged space intelligences. As the film opens, a flying saucer does, indeed, land near the White House lawn, in a baseball field in Washington, D.C. Within minutes, the craft is surrounded by armed military personnel and armored tanks. Klaatu emerges, and as he holds up a gift he has brought for the president, he is shot and wounded by a soldier who misinterprets the alien's gesture as a hostile movement. At this point, Gort, Klaatu's eight-foot robot, leaves the spaceship and fires a kind of laser beam at the assembled military and instantly melts all weapons and armaments. Klaatu halts Gort before it destroys anything—or anyone—else, and the alien's peaceful intentions convince the officers that he has come in peace. Klaatu is taken to a military hospital where his wound can be treated and he can be placed under guard.
Klaatu makes it clear that he has come as an ambassador from an intergalactic federation of planets that has been keeping Earth under surveillance for centuries. Now that Earth's science has advanced to the nuclear age and the planet's influence may soon be extended beyond its own atmosphere, he has been sent to deliver a message of utmost importance to all the heads of state. When Klaatu perceives that his request will be refused, he escapes from the hospital and moves anonymously into a rooming house, posing as a man named Carpenter.
The alien emissary becomes friends with Bobby (Billy Gray) and his mother, Helen (Patricia Neal), and the boy leads him to Professor Barnhardt (Sam Jaffe), a physicist, who is impressed, rather than frightened, by Klaatu's superior knowledge. The scientists in the film are depicted as dedicated individuals who are trying their best to live outside the political bickering and backstabbing of the Cold War era and who are willing to arrange for Klaatu to address an international assemblage of the leaders of world science. Realizing that Earth's heads of state are too chauvinistic to set aside their petty differences and listen to his message, Klaatu arranges a demonstration that no one on the planet will be able to ignore: He shuts off all power and machinery on Earth for one hour.
Considered a threat to national security, Klaatu is killed by the military and his body placed in a cell. Before he was shot, however, he advised Helen what to do if anything should happen to him. She approaches the massive Gort and speaks the order, "Klaatu Barado Nikto," a command that enables the robot to restore life to Klaatu and brings the film to its conclusion and the alien ambassador's final message to all of Earth: "It is your choice. Join us and live in peace or face obliteration." The unsettling implication made by Klaatu before he leaves in his spacecraft is that it really doesn't matter that much to the aliens what earthlings decide.
The admonitions of Klaatu were subsequently repeated in the channelings of the UFO contactees for decades to come. Some critics have made comparisons between Klaatu's mission to Earth and the messages and ministry of Jesus (c. 6 B.C.E.–c. 30 C.E.). Both came from "above"; Jesus was a carpenter, Klaatu chose the alias of Carpenter; both were killed and resurrected by a power beyond Earth's knowledge; both returned to the "heavens" when their message that humans must repent and change their ways had been delivered.