The Day the Earth Stood Still (1951)

Travelling at mind-boggling speed, a gleaming unidentified flying object zooming in from boundless deep space penetrates the Earth's atmosphere, landing smoothly in Cold War Washington, D.C. Encircled by large yet feeble military forces, the peaceful intergalactic ambassador, Klaatu, emerges from the mysterious vessel accompanied by the silently dangerous robot of incomprehensible power, Gort, only to witness firsthand the earthlings' hospitality. The sophisticated humanoid declares that he comes in peace; however, he needs to assemble the world's greatest minds to hear his merciful warning and a definitive ultimatum. Is Klaatu the messenger of humanity's doom?