Chuck Baker is an American astronaut who lands on Planet 51, believing it to be uninhabited. His plan was simple: plant the flag and head back home in his spaceship.

To his great surprise, he discovers that the planet is populated by small green creatures living everyday lives much like humans on Earth. However, they possess a completely paranoid army that believes the planet is about to be invaded by aliens!

Fortunately, Chuck receives excellent help in his quest to return home from his companion robot "Rover," a green friend named Lem, and Lem's companions.