Planet 51 is an animated film directed by Jorge Blanco, written by Joe Stillman and starring Dwayne Johnson, Jessica Biel, Justin Long, Seann William Scott, Gary Oldman and John Cleese. Produced by Madrid-based Ilion Animation Studios and HandMade Films for $60 million, it was acquired for US distribution by New Line Cinema in November 2007. Planet 51 was released on November 20, 2009, by Sony Pictures Worldwide via TriStar Pictures. It was originally titled Planet One, but re-named Planet 51 in reference to Area 51.
The story begins 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 merely a movie, though the plot resembles classic 1950s science fiction films.
Outside the theater is town Clipforg on Planet 51, a world populated by little green aliens who live in a society similar to 1950s USA. Lem (Justin Long) (name is a tribute reference to Stanisław Lem, famous Polish science fiction writer, and could also refer to the Apollo Lunar Excursion Module) is a teenaged alien who has just gotten a job at the local planetarium museum, and his family and friends are excited about it. His best friend Skiff (Seann William Scott), a geeky alien teen who works at the local comic book store, 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), who is Lem's long-time crush. Lem's family is holding a barbecue with the neighbors. Neera is at the barbecue, 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 around Planet 51, 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 artifact "wakes up" from the signal; it is a wheeled probe with artificial intelligence, named "Rover." Rover breaks out of the basement and escapes to perform its primary mission: to find the astronaut. Its secondary mission is to collect rocks, similar to Wall-E gathering garbage for compacting. The army becomes suspicious of Rover's escape and begins to look into its activity.
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 U.S. 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 he's on 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.. Chuck goes into hiding soon after, while the army 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 citizens start a local "civil defense" force to protect the citizens from becoming "zombies."
Lem discovers Chuck hiding at the planetarium, and both are surprised to learn they speak the same language. Chuck introduces himself, and explains he's eager to leave the planet and return home in time for the Nickelodeon Kid's Choice Awards. Lem realizes this "alien" is no threat, and decides to help Chuck, though it costs him his job and a near arrest at the hands of the army. It becomes apparent that Chuck can't go anywhere near his module (which the aliens repeatedly call a "flying saucer"), so Lem hides Chuck in his room for the night.
In the meantime, Rover locates Chuck, though it has to sneak past the soldiers patrolling the neighborhood. Chuck is very happy to see the robot when they reunite, though he tells Rover the probe was not helpful because it only took pictures of rocks and not the little green men living on the planet. (This is what led to the misinformation that the planet was uninhabited). Skiff and Eckle are let in on Lem's secret, as are two soldiers who believe they are being "mind-controlled" by Chuck.
Skiff and Lem decide that the citizens of Planet 51 need to see that Chuck is harmless, and the only way to do that is to have Chuck speak to the media (who have been stoking the fear of the citizenry). So, they boys have two soldiers smuggle Chuck to the comic book store and hide him there, while Lem goes to get the reporters. Before Lem leaves, he asks Chuck to help him with some advice regarding Neera; Chuck attempts to use what he calls "the right stuff," to show Lem how to woo her. In his attempt to find the reporters, Lem has to face Neera, with whom he had a falling out when she and her friends (including Glar) were protesting the army's treatment of the alien and his craft. Chuck's advice in winning Neera back doesn't work well, and there is still friction between the two of them.
Back at the comic book store, Chuck is playing with Eckle, using the whole alien invader theme everyone keeps talking about, while the "mind-controlled" soldiers watch and Skiff bonds with Rover. At one point, Planet 51's signature "rock rain" begins to fall, and Rover becomes ecstatic, since collecting rocks is what he does, and he starts "dancing" in the streets (in reference to Singin' in the Rain).
Unfortunately, Lem and Skiff's plan to peacefully introduce Chuck to the public backfires when reporters discover Chuck in the bookstore and secretly film him quoting lines from Star Wars and The Terminator. That broadcast throws Lem's whole world into chaos, and he, Chuck, and his friends barely escape the soldiers hounding them through town. They hide Chuck in Lem's room again, though it's apparent that things have become decidedly worse. The army sets up barbed wire around the module, and Lem is angry at Chuck for turning his life upside down.
Things get even worse when General Grawl decides to set up a base in Lem's home. Lem and his friends barely escape with Chuck before the general and his men search the Lem's room. The group somehow manages to sneak Chuck into the planetarium, and while there, Chuck admits to Lem that he never had "the right stuff." He says that Lem was the one who had "the right stuff" all along, making risks and sacrifices just for a stranger. Chuck also shows Lem the star that Earth orbits, and how the universe is so much bigger than Lem had thought.
The next morning, the army brings in vehicles to take Chuck's module to a secret location. Even the media isn't allowed to tell where the spacecraft is going.
Chuck is captured and unmasked for all to see, and the General makes a great show of his disgust for the "alien." When Lem tries to defend Chuck and keep him from being taken away, General Grawl merely labels Lem a zombie, and Professor Kipple announces he will dissect both their brains. Chuck is not willing to drag Lem into this, so he pretends to "release" Lem from his control, and the crowd believes it. Both Chuck and Rover are locked into armored vehicles, and when the General makes it clear to Rover that he intends to take him apart piece by piece, the little robot responds like a dog wetting itself in fear, only with oil. They're taken away, and Lem is proclaimed a hero of the community.
At Base 9, General Grawl attempts to get some answers out of a handcuffed Chuck, warning that if he tries taking over the minds of the soldiers in the room, a chain reaction will set off and everyone will shoot at each other and possibly be electrocuted. Chuck accidentally hits a button and starts said chain reaction, though he's not affected by it. General Grawl mistakes this as resisting his demands, and allows Professor Kipple to have a go at dissecting Chuck's brain.
Lem gets his job back at the planetarium and gets to speak on tv, but he cannot truly accept the honor. He feels terrible about Chuck and decides to do the right thing. He hotwires a car, and while he's in the process, Neera comes to talk to him, admiring what he's doing. Neera, Skiff, and Eckle join Lem as they go off to rescue Chuck. However, not only do they not know where the army took him, but they feel they can't find the astronaut without Rover, who had also been taken by the army. But luck is on their side when the little robot shows up on the edge of town, revealing that he unscrewed all the bolts holding the armored vehicle in which he was imprisoned.
In a laboratory, Chuck is strapped to a table while Professor Kipple gets ready to take his brain out. Lem and his friends break in through the ceiling, using Rover to scare away the scientists, technicians and guards. They release Chuck and immediately set off alarms by doing so.
Rover helps the group find Chuck's 'lunar' module, which had been placed in a hangar, but General Grawl stops them. He warns that if the "alien" attempts to leave, the entire hangar will blow up, and reveals the place is rigged with bombs. Lem tries reasoning with Grawl, which in fact is a distraction to set off the bombs and use the confusion to help Chuck escape. Most of the soldiers flee the firestorm, and the General is knocked out amidst the falling debris. Lem, Neera, Eckle, Skiff, and Rover all get into the module, but Chuck decides he can't leave the General to die, and rescues him from the fire.
To escape the growing firestorm, Chuck pilots the ship into orbit around the planet, allowing the alien kids to experience outer space. Kiff, Eckle, and Rover enjoy the weightlessness, General Grawl realizes that Chuck is friendly and hasn't turned him into a zombie, and Lem finally gets to ask Neera out on a date.
Chuck pilots the module back to the planet's surface, and although the army is ready to shoot as everyone walks out, the General stops them from shooting and says that Chuck is with him. Finally the aliens of Planet 51 get to see that Chuck is harmless and never intended to hurt anyone. Chuck lets Rover stay and it becomes Skiff's pet, and Chuck says goodbye to everyone on Planet 51.
As the module is leaving, it's revealed that one of the alien "dogs" snuck on board.