American astronaut Captain Charles "Chuck" Baker, lands on Planet 51 thinking he's the first being to step foot on it. To his surprise, he finds that this planet is inhabited by little green beings who are happily living in a white picket fence world reminiscent of a cheerfully innocent 1950s America, and whose only fear is that it will be overrun by alien invaders--like Chuck! With the help of his robot companion "Rover" and his new friend Lem, Chuck must navigate his way through the dazzling, but bewildering, landscape of Planet 51 to find his way to his own planet, Earth.