Produced in the wake of World War II, with the world leery about atomic bombs in the early days of the Cold War, The Day the Earth Stood Still is a classic 1951 sci-fi film released by 20th Century Fox that tells the story of a humanoid-alien who comes to Earth to warn humanity not to take their conflicts into space, lest they will face devastating consequences. The film strongly and iconically addresses issues of violence, politics, and the fear of global annihilation.
Klaatu’s spaceship lands in Washington, DC. where he declares that he has come in peace on a mission of goodwill, however when he activates a small device that opens with a snap, Klaatu is shot and wounded by a nervous soldier. In response, a large robot called Gort steps out of the spaceship and melts all weapons present without harming the soldiers, until the wounded Klaatu orders him to cease. Klaatu explains the "weapon" was a gift to the President and could have been used to study the other planets.
Taken to Walter Reed Hospital to recover, Klaatu is rebuffed in his attempt to organize a meeting of world leaders, instead he is held prisoner; although he later easily escapes into the night.
Befriending the residents of a boarding house, Klaatu endeavors to understand humans through this interaction and is told by a young boy, Bobby, that the greatest person in the world today is leading American scientist, Professor Barnhardt, who lives nearby. When the professor is absent, Klaatu solves an advanced mathematical problem written on a blackboard, before leaving his address with the housekeeper. Later, Klaatu warns the professor that other planets know that human beings have developed atomic power and will not allow this to endanger them. Klaatu accepts an offer to speak at a meeting of world scientists that Barnhardt is organizing, but the scientist is stunned when Klaatu declares that if his message is rejected by the leaders of the nations, "Planet Earth will be eliminated." The professor soberly pleads that Klaatu first provide a minor demonstration of his power as a warning.
Following a worldwide demonstration of his vast power, Klaatu is shot dead by the army. Bobby’s mother Helen follows his earlier instruction to tell the robot Gort, "Klaatu barada nikto," if anything had happened to him.
Gort then retrieves Klaatu's corpse, brings him back to the spaceship, and using equipment on board, miraculously restores him to life. Klaatu steps out of the spaceship and speaks to the assembled scientists. He tells them that Earth's penchant for violence and first steps into space have drawn the attention of other space-faring worlds. These worlds have created a race of robot enforcers like Gort, and given them absolute power to deal with any outbreak of violence. He warns that Earth can either abandon warfare and peacefully join other space-faring powers – or be totally destroyed. "The decision rests with you." He then enters the spaceship and departs.
Trivia: The post title is a quote spoken by Klaatu (Michael Rennie) during the films clmactic speech.