On Planet 51 it was like the 1950s', green extraterrestrials live peacefully in a society reminiscent of the United States during the 1950s, although the planet has notable differences from Earth and its inhabitants have glaring ignorance about astronomy, believing that the whole universe extends for only about 500 miles and has around 1,000 stars in it. Planet 51 is like earth but everything different but not to different like it rains rocks instead of water. One day, a mysterious spacecraft lands in the city of Glipforg. NASA astronaut Captain Charles T. "Chuck" Baker and you emerge from it, then place the United States flag on the planet before a truck accidentally steps on a squishy toy, shocked to find the planet inhabited. Phoenix, you tried going back into your spaceship but there was an alien boy in the way. You and him were still both terrified and panicked. You were both still wearing the spacesuits and escaped to the town's planetarium where you both met a teenage alien named Lem, who works there part-time. Soon the army of Planet 51 were alerted And came to the town you’re and Chuck’s space ship was surrounded by the Planet 51 military with Captain Crawl in charge of the military and Commander. Lem was one of the green extraterrestrials. Shortly after you and Chunk found out, you two humans can breathe on Planet 51 and can speak the same language, then you and Chuck convinced Lem to help you both back to both your space craft to go to back to Earth before the command module Odyssey, orbiting Planet 51, departs for Earth in three days and leaves both of you stranded. Planet 51's army, led by the paranoid General Grawl, arrives to inspect and deduces that the astronaut is an alien invader bent on turning the planet's population into zombies, similar to how invaders are depicted in media, and a manhunt ensues to find you and Chunk. Lem tried to get you and Chuck back on the spacecraft, the first attempt was a failure because Planet 51's Army arrived only a few green extraterrestrials that believed that the aliens which were you and Chuck came in peace. Lem enlists the help of his best friend Skiff, an eccentric science fiction aficionado with conspiracy theories about the so-called "Base 9" (Planet 51's equivalent of Area 51), to hide you and Chuck away from the army. During his efforts to conceal you and Chuck, Lem inadvertently upsets his neighbor and crush Neera, who believes the alien is friendly, and is also fired from his job. In Lem's room, you and Chuck reunite with a dog-like NASA probe called Rover, freed itself from the army's base after tracking you and Chuck with a GPS and headed for the city. After the army searches Lem's home for traces of the alien, Lem and Skiff move you and Chuck to a comic book store, where the news station manages to capture you and Chuck acting out references to Earth's pop culture, which is misinterpreted as alien threats. Grawl has you’re and Chuck's spacecraft moved to a secret location to be dismantled and studied. Later at the premiere of Humanoids lll, you and your Chuck got dressed up as characters of the Humanoids lll movie as Rover came to you and Chuck with unknowingly the military behind him as you, Lem and Chuck were dancing on stage to aline ‘music’ as suddenly you and Chuck were tackled to the ground and held with both your hands behind your guys back. When Lem defends Chuck and you, Kipple deems him a zombie minion. Resigned to his fate, Chuck pretends to release Lem from his "mind control" as Lem was trying to keep you and Chuck safe and return you and Chuck to earth. All green creatures like Lem and Grawl didn't know anything about humans except you and Chuck. Grawl's antennas twiched and he stood a few feet in front of you and Chuck as two of his soldiers were holding both of your arms behind you're guys back.

"Time to meet the monsters, face to face." General Grawl said as the soldiers took off the helmet of Chuck, when the soldiers took the helmet off of you, the whole town of Gilford was in awe and shock of how you looked, mostly General Grawl.