An alien, Klatuu, lands on earth to warn us that we must live peacefully, or we will be destoyed as a threat to other planets. He is wounded and taken to hospital, from which he escapes, and he goes on an under-cover mission to discover more about the planet, with his trust robot, Gort, ready at his side.
This 1951 classic is memorable for the iconic image of the flying saucer on the White House lawn, the giant zapping robot with the mysterious override code ("Klaatu barada nikto"), and Michael Rennie as a civilised alien visitor who exposes hypocrisy and delivers an anti-war ultimatum as chilling as it is pertinent. There's also a general feeling of humane seriousness from a genre and an industry more usually committed to the lurid.
A welcome dose of humane seriousness in a Hollywood world usually more comitted to the lurid.