If we land on another planet -- or back here on Earth -- and find something other than what we expect, who is the alien in that situation, the modern day Earthman, or the local? It's a plot staple of the classic old Twilight Zone series and the basis of a lot of films, including the entire Planet of the Apes series. It's the allegorical mirror of Pogo's famous "we have met the enemy and he is us."
This is also the starting point for the lackluster but amusing Planet 51, starring the voice talents of Dwayne Johnson (as NASA captain Charles Baker), Justin Long (as the boy Lem), Jessica Biel (as Neera), Gary Oldman (General Grawl) and John Cleese (as dorky Professor Kipple). It seems NASA has mastered galactic flight and has sent the dashing Charles Baker across the universe to explore an uninhabited, but Earth-like planet.
Later we find he's following the Wall-E inspired "Rover" probe, which vanished once it landed on the planet, but at the beginning the film starts by showing us an idyllic suburban small town a la Hill Valley in Back to the Future. The setting is mostly the 1950's (hence the "51" in Planet 51, presumably, because it's hard to imagine a stable solar system with over fifty planets) though there's a bit of 60's hippie bleedover, including a VW van covered in peace slogans and the long-haired teen rebel Glar (Alan Marriott).
Ultimately, though, the sight gags and light storyline of an alien populace terrified of invasion and the table-turning irony of it being an Earthman -- one of "us" -- who ends up being the invader from another planet are not enough to sustain this pretty, but vacant children's fare. That's why I'd suggest this is a good DVD rental with its bright colors and bouncy narrative, but not worth $20+ of your hard earned cash to take the kids to a matinee.
With any movie, it's always interesting to keep asking "why?" as it unspools. In the case of Planet 51, why have it set in a retro 1950s was the one question that stuck with me, and the answer I finally settled on wasn't the visual style or the fact that we as a nation were wrestling with the possibility of men in space, but that it was during the mid 50's that the Soviets launched Sputnik and completely changed the Cold War, dramatically escalating tensions and creating a period that was more characterized by fear of "the other" than anything hopeful.
In a similar manner, the fear of an alien invasion is unquestionably the zeitgeist of the inhabitants of Planet 51, even as the government has a top-secret research base where alien artifacts are stored. Called Area Nine, it's an obvious (to adults) riff on the paranoid theories about the U. S. Air Force's mysterious Area 51. Both are located in the desert: the real one is supposedly in nowhere, Nevada. In fact, now that I think about it, perhaps the "51" in the title comes from Area 51 after all. Perhaps someone should Google it and post a comment to clarify?
As an unabashed fan of 50s and 60s sci-fi films, most of which dealt with the fear of alien invaders of one sort or another (ranging from The Day the Earth Stood Still to The Thing to War of the Worlds to Invasion of the Body Snatchers), I loved that Planet 51's residents were similarly obsessed with Humanoids and fanatically devoted to a series of movies all about those scary human invaders who wanted to turn them into zombies, enslave them, and eat them.
The lost NASA probe "Rover" looks one heck of a lot like Wall-E, doesn't it?
There was lots of potential in the voice casting too, particularly the talented Gary Oldman as the forecful General Grawl. It was a chance to have him revisit his memorable (and memorably named!) character Jean-Baptiste Emanuel Zorg from The Fifth Element, but they gave him no space to explore the character and, as with too many of the characterizations in the film, his was uninteresting.
This was also the case with John Cleese, a brilliant comic actor who has been focused on his voice, timing, and pacing for decades. As the squirrelly little mad scientist Professor Kipple, constantly roaming around wanting to cut up Baker's skull so he could examine his brain (which is a nice reflective riff on the zombie fears everyone else in the story has about what the aliens would do if they land), Cleese could have had a stand out role and brought much humor. That he doesn't is the indicator of just what went wrong with Planet 51: talent frittered away, too many sight gags, too much effort spent on visuals in lieu of thoughtful story development.
As I said earlier, I'd suggest you just wait until it's out on DVD and then rent it - or stream it from a service like Netflix - because it's not a bad movie. It's just not worth more than a few bucks to see it.