On Planet 51, green humanoids with snail-like feelers and pointed ears live peacefully in a society reminiscent of the 1950s United States, but with alien technology and 1950s-themed alien homes. In the town of Glipforg, Lem is a teenager with a new part-time job at the local planetarium and a long-time crush on his neighbor, Neera. His best friend is Skiff, a big fan of the Humaniacs films. Lem tries to ask Neera on a date, but her hippie friend, Glar, keeps interrupting with his protest songs.
A mysterious spacecraft goes into orbit around Planet 51 and sends out a signal. On Planet 51, under a secret Army installation called Base 9 (their version of Area 51), there is a basement filled with Earth artifacts including robotic satellites launched by the United States and the Soviet Union. The spacecraft's signal activates Rover, a wheeled A.I. probe. Rover escapes, following a program to locate the newly arrived astronaut. The Planetary Army becomes suspicious after Rover's escape and begins to investigate.
The spacecraft touches down in Neera's backyard; NASA astronaut Charles "Chuck" T. Baker emerges. As Chuck plants the United States flag, he steps on a "rubber ducky" someone left on the ground. Everyone stares, and Chuck tries to get back into his module. Shocked when he realizes he is on an inhabited planet, Chuck runs about wildly and hides at the planetarium as the Army arrives on the scene. Planetary Army General Grawl consults Professor Kipple about the "alien invader". The Army quarantines the area and asks citizens to start a local civil defense force to prevent the citizens from becoming "zombies".
At the planetarium, Lem discovers Chuck's hiding place. They are surprised to discover they speak the same language. Realizing this alien is no threat, Lem decides to help Chuck, hiding him in his bedroom for the night. Rover finds Chuck, who is very happy to see it. As the General and his men search Lem's room, the group sneaks back into the planetarium with Chuck, who tells Lem he has "the right stuff". Chuck also shows Lem the star that Earth orbits and how the universe is much bigger than Lem had thought. The next morning, the Army takes Chuck's module to a secret location.
The next night is the première of a new Humaniacs movie, and Lem, Skiff and Chuck attend in costume as fans. Chuck introduces some new music as part of the contest and teaches Lem to dance. Things go well until Rover arrives: chaos ensues. Grawl arrives and points out that Chuck's U.S. flag insignia identifies him as the alien. Chuck is captured and unmasked. When Lem tries to keep him from being taken away, Grawl labels Lem a zombie, and Kipple announces he will dissect both their brains. To protect Lem, Chuck pretends to "release" him from his control. Lem is proclaimed a hero, and Chuck and Rover are taken away.
At Base 9, Grawl interrogates Chuck, and an accidental gunshot starts a complicated chain reaction with soldiers shooting at each other. Grawl thinks Chuck is resisting his demands and allows Kipple to have a go at dissecting Chuck's brain. Lem gets his job back at the planetarium and is permitted to speak about the incident on TV, but cannot fully accept the honor. He feels terrible about Chuck and decides to do the right thing. Neera, Skiff and Eckle join him to go off to rescue Chuck. The teens know luck is on their side when Rover shows up on the edge of town—he unscrewed the bolts holding together the armored vehicle in which he was imprisoned.
Rover sniffs out Chuck's trail and leads them to an abandoned gas station in the desert. They look around, and Skiff sees a soda bottle in an old refrigerator. When he tries to take it, it opens up the entrance to Base 9. Lem has Glar distract the soldiers guarding the base with his protest group while the rest of them sneak into the base. They find Chuck strapped to a laboratory table, and Kipple getting ready to remove his brain. Lem and his friends break in through the ceiling while Rover scares away the scientists, technicians, and guards. They release Chuck but set off alarms.
Rover helps the group find Chuck's spacecraft, which was in a hangar. Grawl warns that, if the "alien" tries to leave, he will blow up the hangar. To give Chuck his chance to escape, Lem initiates the 2-minute destruct countdown by pressing the self-destruct control button, which panics the guards into fleeing the underground base. In the resulting firestorm, the General is knocked unconscious. The teenagers and Rover get into the module, but Chuck rescues the General from the fire. Chuck pilots the ship out of the base just in time as the base starts to blow up and up into orbit around the planet, allowing the teens to experience outer space. Skiff and Eckle enjoy the weightlessness, Grawl realizes Chuck has not turned him into a zombie, and Lem asks Neera out on a date. Chuck returns to the planet's surface. Although the soldiers are ready to shoot anyone who appears, Grawl stops them. Finally, the inhabitants of Planet 51 see that Chuck came to their planet for peaceful purposes. Chuck lets Rover stay with Skiff and says his farewells to Planet 51. Chuck then leaves Planet 51 peacefully, accompanied by the alien pet dog seen throughout the film (who had sneaked aboard Chuck's ship without anyone noticing, except its owner).
In an end scene Kipple is seen emerging from Base 9 from a hatch, only to be seized and dragged away by two military test subjects of Kipple's brain removal procedure who insist that the professor share the delightful experience of having his brain removed.Dwayne Johnson as Captain Charles "Chuck" T. Baker, a NASA human astronaut.
Justin Long as Lem, a teenage boy living on Planet 51.
Jessica Biel as Neera, a teenage girl and Lem's love interest.
Gary Oldman as General Grawl, afraid of an alien invasion and convinced that Chuck is an evil, mind-controlling alien who turns the planet's inhabitants into "alien zombies".
Seann William Scott as Skiff, Lem's best friend, who works at a comic store.
John Cleese as Professor Kipple, a scientist on Planet 51, determined to study Chuck's brain.
James Corden and Mathew Horne as two rather gullible soldiers who believe they have been turned into zombies.
Freddie Benedict as Eckle, Neera's younger brother who is a huge fan of the "Humaniacs" comic books and movies
Alan Marriott as Glar, a teenage hippie who is into protesting and can play the guitar.
Planet 51 is based on the original idea by Jorge Blanco, Marcos Martínez, Ignacio Pérez Dolset and Javier Abad. The film finished production by June 2009.
The name change from Planet One to Planet 51 was a result of the demands made from another entity branded Planet One, which produces children and teen TV programmes. They made contact with the film's producers early on to resolve the trademark and brand confusion issues. The Spanish film company made an offer to Planet One for all ownership rights to their "Planet One" trademarks and related website URLs. Planet One chose not to take that offer and to protect their brand and trademarks that have been active for many years. As a result, the film's producers chose to rename the film Planet 51. This is a reference to the high top secret military base Area 51 where conspiracy theorists claim that data and specimens from a space alien landed on Earth in 1947 are stored.
Lem was named by screenwriter Joe Stillman after Polish science fiction writer Stanisław Lem. Since the film was intended to be a parody of American pulp science fiction shot in Eastern Europe, Stillman thought it would be hilarious to hint at the writer whose works have nothing to do with little green men (Stanisław Lem is known to be highly critical towards American science fiction).
In November 2007, New Line Cinema had picked up the United States distribution rights; the studio itself was to release the film in the summer of 2009. However, TriStar Pictures became the film's home after New Line Cinema sold the rights to them through Sony Pictures Worldwide Acquisitions Group. According to the Variety magazine, New Line Cinema's owner, Warner, "decided to let the pic go after the producers insisted on a November release, when Warner is releasing its sixth Harry Potter pic." The new distributor moved the U.S release date from the summer of 2009 to November of that year.
The film was released by Sony Pictures Home Entertainment on Blu-ray and DVD on 9 March 2010.
The film was released in 3,035 cinemas, grossing $3.2 million on its opening day and $12.6 million over the weekend, resulting in the number four position at the box office behind 2012, The Blind Side and The Twilight Saga: New Moon respectively. During its theatrical run, it made over $42 million, with a total of $105 million worldwide.
Rotten Tomatoes reported that 21% of critics gave Planet 51 positive reviews based on 107 reviews with an average score of 4.2/10. The site's consensus reads: "Planet 51 squanders an interesting premise with an overly familiar storyline, stock characters, and humor that alternates between curious and potentially offensive." Another review aggregator, Metacritic, which assigns a rating from 100 top reviews from mainstream critics, gave it an average score of 39% based on 21 reviews.
Adam Markovitz of Entertainment Weekly graded the film a B, as it "delivers a few pleasant surprises, including a smart story". Roger Ebert of the Chicago Sun-Times gave 2½ stars out of 4 and positively wrote of the film being "perfectly pleasant as kiddie entertainment, although wall-to-wall with pop references to the American 1950s." However, some critics such as Markovitz, Steven Rea of The Philadelphia Inquirer, and Brian Miller of The Village Voice acknowledged Planet 51 as "an E.T. in reverse".
The soundtrack album for the film was released by Decca Label Group on 10 November 2009 (digital) and 17 November 2009 (CD).
A video game based on the film was announced in November 2009. The game was published by Sega and was released on Nintendo DS, Wii, Xbox 360 and PlayStation 3 on 17 November 2009. The console versions was developed by Pyro Studios and the Nintendo DS version was developed by Firebrand Games. Zed Group, a long-time customer of Trinigy's, worked on the online version of the game with the Vision Engine. There are also Planet 51 games for iPhone, mobile devices and Facebook, developed by Zed Worldwide, which belongs to the Ilion's owner's brother.