On a distant, green, and bustling planet called Planet 51, life unfolds in a peaceful and retro atmosphere, very similar to that of the 1950s on Earth. The adorable inhabitants of this planet lead a quiet life, enjoying their workdays and outdoor hobbies, unaware of the presence of extraterrestrials. However, everything changes when a human astronaut named Chuck Baker accidentally lands in their world.
Chuck, excited to discover a new home, finds himself in a scenario very different from what he expected: the aliens think he is a threat and that his arrival is an invasion. Teeming with distrust, the brave Lem, a young inhabitant of Planet 51, decides to help Chuck avoid capture and find a way back home. Meanwhile, the local government, led by the fearful General Grawl, is determined to capture the human intruder before he disrupts the peace of their planet.
Together, Chuck and Lem embark on a series of fun adventures, trying to evade the authorities and experiencing the cultural clash between two completely different worlds. As time runs against them, both friends discover the value of friendship, understanding, and tolerance. In an exciting conclusion, they will learn that being different does not mean being an enemy, but rather that diversity can be the key to true intergalactic peace.