On Planet 51, green humanoids with snail-like feelers and pointed ears live peacefully in a society reminiscent of the 1950s United States, but with alien technology and 1950s-themed alien homes. In the town of Glipforg, Lem is a teenager with a new part-time job at the local planetarium and a long-time crush on his neighbor, Neera. His best friend is Skiff, a big fan of the Humaniacs films. Lem tries to ask Neera on a date, but her hippie friend, Glar, keeps interrupting with his protest songs.
A mysterious spacecraft goes into orbit around Planet 51 and sends out a signal. On Planet 51, under a secret Army installation called Base 9 (their version of Area 51), there is a basement filled with Earth artifacts including robotic satellites launched by the United States and the Soviet Union. The spacecraft’s signal activates Rover, a wheeled A.I. probe. Rover escapes, following a program to locate the newly arrived astronaut. The Planetary Army becomes suspicious after Rover’s escape and begins to investigate.
The spacecraft touches down in Lem’s backyard; NASA astronaut Charles “Chuck” T. Baker emerges. As Chuck plants the United States flag, he steps on a “rubber ducky” someone left on the ground. Everyone stares, and Chuck tries to get back into his module. Shocked when he realizes he is on an inhabited planet, Chuck runs about wildly and hides at the planetarium as the Army arrives on the scene. Planetary Army General Grawl consults Professor Kipple about the “alien invader”. The Army quarantines the area and asks citizens to start a local civil defense force to prevent the citizens from becoming “zombies”.
At the planetarium, Lem discovers Chuck’s hiding place. They are surprised to discover they speak the same language. Realizing this alien is no threat, Lem decides to help Chuck, hiding him in his bedroom for the night. Rover finds Chuck, who is very happy to see it. As the General and his men search Lem’s room, the group sneaks back into the planetarium with Chuck, who tells Lem he has “the right stuff”. Chuck also shows Lem the star that Earth orbits and how the universe is much bigger than Lem had thought. The next morning, the Army takes Chuck’s module to a secret location.