Plot

On Planet 51, green humanoids with snail-like feelers and pointed ears live peacefully in a society reminiscent of 1950s America, but with alien technology and with 1950s-themed alien homes. In the town of Glipforg, Lem is a teenage boy with a new part-time job at the local planetarium and a long-time crush on his neighbour Neera.

His best friend is Skiff, a big fan of the Humaniacs films.  At a barbecue Neera’s family is having, Lem tries to ask her on a date, but her hippie friend, Glar, keeps interrupting with his protest songs.

Just then, 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 artefacts 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.