* Dwayne Johnson as Captain Charles "Chuck" Baker * Jessica Biel as Neera * Justin Long as Lem and Rover (vocal effects) * Seann William Scott as Skiff * Gary Oldman as General Grawl * John Cleese as Professer Kripple * Ryan Rosenfeld as Marv Plot
The story starts with two teenagers being attacked by a "humaniac," a horrendous one-eyed monster that comes down to the planet to turn everyone into zombies and take over the world. Then it is revealed that this is a mere movie, though the plotline resembles classical 1950s science fiction films.
Outside the theater is Planet 51, a world populated by little green aliens that live in a society similar to 1950s earth. Lem (Justin Long) is a teenage alien who has just gotten a job working at the local planetarium museum, and everyone in his neighborhood is excited about it. His best friend is Skiff (Seann William Scott), a geeky alien teen who works at the local comic book store, and is a huge fan of the "Humaniacs" films.
When Lem gets home, he asks his next door neighbor and friend, Eckle (Freddie Benedict) about Neera (Jessica Biel), a long-time crush of his. His family is holding a barbecue with the neighbors, so Neera is there, but when Lem attempts to ask her out, he keeps being interrupted by Neera's hippie friend, Glar.
Meanwhile, a mysterious spacecraft pulls into orbit, sending out a blinking red signal. Down on the planet, under a hidden army base called "Base 9" (the aliens' version of Area 51), there is a basement filled with artifacts from earth. One of them "wakes up" from the signal, and is a wheeled probe with an AI named "Rover." It breaks out of the basement and escapes, off to perform its primary mission: find astronaut. Its secondary mission is to collect rocks, (similar to Wall-E gathering garbage for compacting). Due to Rover's escape, the army becomes very suspicious of what's going on.
Meanwhile, just as the barbecue starts, a spacecraft similar to the lunar module touches down right in Lem's backyard. The astronaut (Dwayne Johnson), Charles 'Chuck' Baker, emerges, humming the theme from 2001: A Space Odyssey. As Chuck plants the flag, he's startled to step on an alien "rubber ducky" someone left on the ground. Everyone stares, and Chuck attempts to get back into his module - but Eckle is in his way, eager for an autograph. Chuck ends up running about, shocked at the revelation that this is an inhabited planet. At one point, he gets hit by the mailman's scooter in the middle of the street and goes flying past the moon (in reference to E.T.). He goes into hiding soon after, while the army immediately arrives on the scene.
General Grawl (Gary Oldman) is at the head of the investigation, and consults Professor Kipple (John Cleese) on intelligence about the "alien invader." The army then quarantines the area and has locals start a local "civil defense" force to protect the citizens from becoming "zombies."
Up at the planetarium, Lem discovers Chuck hiding there, and both are surprised to learn that they speak the same language. Chuck introduces himself, and he's eager to get off the planet and get back home in time for the Nickelodeon Kid's Choice Awards. It's then that Lem realizes the "alien" is no threat and decides to help him, though it costs him his job and a near arrest at the hands of the army. It soon becomes apparent that Chuck can't go anywhere near his module (which the aliens repeatedly call a "flying saucer"), so Lem hides him in his room for the night. http://www.imdb.com/title/tt0762125/