Actually, this is about The Day the Earth Stood Still (2008), but my version of the title summarizes the movie nicely. It's a remake of The Day the Earth Stood Still (1951), which I have not seen, but that I'm given to understand is about aliens who want to save the Earth from humans destroying it through war. In the new version, Reeves plays an alien (in a convenient human body) who's come to the Earth to save it from humans who are destroying it with their technology.
Although Keano apparently had all his emotions removed after the Bill & Ted movies, since he's playing an alien who is new to a human body, that actually works for him, and he does manage to show a little bit toward the end of the movie.
Kathy Bates turns in a reasonable performance as the Secretary of Defense, initially paranoid and warlike, but eventually coming around. And John Cleese has a rare but enjoyable non-comedic role as a scientist, who helps to convince Klaatu that humans might be able to change, but only if they've given a chance to try when things look darkest.
The real star of the movie was the special effects - there are huge glowing space marbles, and Gort, who looks like a giant Cylon, but turns out to be made up of zillions of microscopic metal (but somehow still organic) bugs. Seeing those bugs rip up a military base, a truck, and finally Central Park are the really entertaining parts of the movie. Seeing Will Smith's son Jaden hate and then warm up to the alien I could have done without, as could the movie in general.
The ending I'm not sure about. Klaatu is finally convinced that humans, on the edge of extinction, may be able to change, so instead of destroying the world, he apparently deactivates all our technology. Cars come to a stop, computers shut down, oil wells stop pumping, and even electric watches stop working. The problem I have with this is that if all our technology just quit, billions of people would die, as the technology to deliver their food would no longer function. Could we develop new technology fast enough to save any of us, or would we revert to pre-industrial tech and have to start all over?
But, the movie did leave me thinking at the end, and the visuals were absolutely stunning. Catch it in the theater some afternoon you have nothing better to do; or if not, then rent it when it hits DVD.