The Day the Earth Stood Still (1951, 92m,20th Cent. Fox), starring Michael Rennie as "Klaatu", with Patricia Neal. An alien comes to Earth, shows us his power about shutting down all of our power for a day, and then said for us to shape up, or his large robot Gort will destroy the planet. We promise to fly right, and he leaves. (I guess we fooled him). Gave us the classic phrase "Klaatu Barado Nicto" which means basically, "Klaatu says 'Don't destroy the planet'" Also, Klaatu wasn't ill that day, but actually stuck in an elevator. Directed by Robert Wise, who would later direct West Side Story, The Sound of Music and Star Trek: The Motion Picture and then become president of the Academy of Motion Picture Arts and Sciences (the Oscar™ people). The movie was inspired by a short story called "Farewell to the Master" by Harry Bates (1940) which has little in common with the movie, other than having a flying saucer piloted by aliens named Klaatu & Gort land in Washington DC, but is just as a thought provoking story as the film is.

P.S. - As an example of what a fast-food world we like in, that last link for the Academy of Motion Picture Arts and Sciences  was originally to www.ampas.org.  However, the URL now no longer works -- apparently the AMPAS has abandoned it own name, in favor of it's cool nickname, www.oscars.org.