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, an alien teenager named Lem (Justin Long) quickly realizes that Chuck is friendly and the two blast off on an adventure to try and return Chuck safely to his space ship!