They're Here Already! You're Next!

Can't you see? They're after you. They're after all of us.

The climax of one of the best sci-fi movies of all time. It's always been considered to be a thinly-veiled criticism of the McCarthy era (which it certainly was), but it's a pretty malleable metaphor for lotsa things: conformity, religious extremism, fascism, hyper-commercialism. Take your pick.