When Chuck the astronaut (Dwayne Johnson) lands on a distant planet filled with little green people, he is surprised to discover that we are not alone in the galaxy. But he gets the shock of his life when the residents of Planet 51 mistakenly believe that his presence is the start of an alien invasion of the human kind! Luckily, Lem (Justin Long) quickly realizes that Chuck is friendly and makes it his personal mission to help him return safely to his ship.