Sixteen year old Lem lives on Planet 51, a white picket fenced world reminiscent of 1950's America. His life is happy, safe and predictable... until the unpredictable happens! Out of the blue, astronaut Captain Charles 'Chuck' Baker's spaceship comes crashing down right in the middle of a family barbecue! Unaware of his surroundings, Chuck is revelling in his "Big Moment"; he plants his flag triumphantly before realizing he's not alone. Turning, he sees Planet 51's green-skinned inhabitants staring back at him bemused! In a panic he flees, bumping into many of the panicked natives who find this "alien" creature equally as frightening.