AA flying saucer lands in Washington, D.C. carrying a humanlike spaceman named Klaatu (Michael Rennie) and a large and powerful robot named Gort. Moments after emerging from the spaceship, Klaatu is shot and wounded by a nervous US Army soldier. Gort begins to retaliate by vaporizing all of the army weaponry (guns, tanks, etc) at the landing site, until Klaatu stops him. Klaatu is taken to a nearby military hospital where he makes a rapid recovery. He tries to explain to one of the President's top aides that it is imperative he be allowed to address all of the nations of the Earth, but his request is declined. Rather than be detained at the hospital, Klaatu escapes and, under the name of Mr. Carpenter, he takes a room at a boarding house in the city where he befriends widow Helen Benson (Patricia Neal) and her young son Bobby (Billy Gray). With Bobby's help, Klaatu visits Professor Jacob Barnhardt (Sam Jaffe), one of the world's most eminent scientists. Klaatu explains to the professor that his mission is to deliver a warning and an ultimatum to the people of the Earth about their warlike ways and the misuse of atomic energy. Should his warning not be heeded, it will mean that the planet Earth and all of its inhabitants will be eliminated.