A kid called Genius accidentally invents a substance that causes all animal life to grow. Genius' sister Nancy and her cardigan-wearing boyfriend Mike see the money-making potential in the substance (dubbed "Goo") and plan to get rich by selling it.
A greedy young opportunist and his delinquent friends learn about the substance and are inspired to steal the Goo and use it for their own purposes. They soon rashly decide to eat it themselves. They grow to gigantic proportions and temporarily take over the town. The giants are opposed by Mike and his friends until Genius can develop an antidote and the giant teens are returned to normal size.
- Tommy Kirk as Mike
- Johnny Crawford as Horsey
- Beau Bridges as Fred
- Ron Howard as Genius
- Charla Doherty as Nancy
- Jim Begg as Fatso
- Joy Harmon as Merrie
- Robert Random as Rick
- Joe Turkel as Sheriff
- The Beau Brummels and Freddy "Boom Boom" Cannon make musical guest appearances.
- As the giant teens rampage through residential streets, some recognizable homes from TV series can be seen, including the Cleaver’s house from Leave It To Beaver and the Stevens’ house from Bewitched.
- Several of the outdoor shots during the showdown between tiny Tommy Kirk and big Beau Bridges were filmed on the "Courthouse Square" location later made famous in the Back to the Future series.
- The red-headed dancer is played by Toni Basil, who later became well-known for her 1982 hit song "Hey Mickey."