A spaceship lands in Washington, D.C., causing a global sensation and raising tensions. Emerging from the craft, a humanoid alien named Klaatu and his powerful robot companion, Gort, are met with hostility from the military. Klaatu, who speaks flawless English, announces that he has a message of grave importance to deliver to the leaders of all nations. However, political barriers make such a meeting impossible. Injured during his initial capture attempt, Klaatu is taken to a hospital but quickly escapes, going undercover to learn more about human society.

Klaatu lodges at a boarding house while being sought after by military and government forces. During his stay, he befriends a widow, Helen Benson, and her young son, Bobby. Through their interactions, he gathers insight into humanity's potential both for violence and compassion. His search for someone capable of understanding his urgent message leads him to a respected scientist, Professor Barnhardt, who becomes an ally. As time runs out, Klaatu enlists Helen and Bobby's help to orchestrate a dramatic demonstration intended to ensure that his warning about Earth's potential self-destruction is heeded by all of humanity.