The movie starts with the landing of a spaceship in Washington, D.C. Since this is obviously a reason for some concern, the army surrounds it and waits for something to come out. When an alien does come out, he's carrying some sort of metal thing that pops open suddenly, one of the soldiers panics and shoots the alien (Whose name, by the way, is Klaatu.) A giant robot, Gort, then comes out of the ship and starts shooting at people until everything calms down. The alien is taken to a hospital, and the robot stays where it is. In the hospital Klaatu says that he wants to deliver a message to all of the leaders of the world in one place. This isn't actually possible, because it's 1951 and probably the worst time he could have asked for that, so instead Klaatu escapes from the hospital and goes out into the world to learn about humans. He befriends a small child, learns about the world, meets a scientist, etc. Eventually he decides to gather up all of the world's scientific leaders and deliver his message, and to demonstrate his power, he cuts off all of the power in the world for half an hour. The government take this as a threat and shoots him. His robot brings him back to life and he delivers his message.
Apparently, there is a sort of galactic peace-keeping organization run through robots. If anything at all violent happens, the robots start killing everyone in sight. You know, for peace. Planets are therefore forced to be peaceful, least they get destroyed by robots. Since humanity is on there way to space travel, they have to stop their warring ways or be destroyed in the name of stopping violence. After all of the movie, the message the alien had was basically just a heads up. Huh. The poster doesn't actually have a lot to do with the actual movie, but it does portray the spirit of it quite accurately.
This is a fun movie. It's cheesy and the robot is just a guy in a suit and the message doesn't really make sense (Fight violence with violence?), but that's what makes the movie fun. It's so 50's and awesome. On the other hand, though, it's also iconic, a big step for science fiction, and has a serious message. I can totally see why it made the list although I don't know if I would have put it as high as it is. The 50's science fiction movies are a big subset of the genre. They're like really long Twilight Zone episodes. I enjoyed watching the movie, even though I kind of enjoyed it because it was so wonderfully cheesy. It's a classic.