An alien dignitary, Klaatu, has come to Earth with his deadly robot, Gort, to deliver a message that earthlings must stop warring amongst themselves - or else. After being shot by military guards, Klaatu is brought to a Washington D.C. hospital, where he begs a sympathetic but frank Major to gather all the world's leaders so he can tell them more specifically what he has come to warn them about. Losing patience, Klaatu slips into the human world, adapting a false identity and living at a boarding house where he meets a smart woman with a conscience and her inquisitive son. Both mother and son soon find themselves embroiled in the complex mystery of Klaatu, his message and the government's witch hunt for the alien.