Which 1951 science fiction film features the warning 'Klaatu barada nikto'?

The 1951 science fiction film featuring the warning “Klaatu barada nikto” is The Day the Earth Stood Still. The phrase is spoken during Klaatu’s mission to Earth, when he instructs Helen Benson to repeat it to the powerful robot Gort if anything happens to him.

Robert Wise directed the film for 20th Century Fox, with Michael Rennie playing Klaatu and Patricia Neal portraying Helen Benson. Klaatu arrives in Washington, D.C., in a flying saucer with Gort, intending to deliver a warning about humanity’s dangerous use of atomic weapons and its growing ability to threaten other worlds.

The phrase has never received an official translation. Its significance comes from the film’s plot: after Klaatu is shot and killed, Helen says the words to Gort, who retrieves Klaatu’s body and revives him. The movie is often remembered as a straightforward alien-invasion story, but its central message is closer to a Cold War parable about militarism, international cooperation, and the consequences of aggression. The 2008 remake also used the phrase.