On the outskirts of the small village of Hainesville, a landslide has closed off the community. As the storm rages, a car full of drunken teenagers has crashed into the landslide. Throwing open the doors, the music from the car radio blares out the doors as they dance and drink beer in the rain. After some time, one of them notices a broken sign telling that Hainesville is 3 miles away. One of them suggests to go there to have some fun.
In town, teenage couple Mike (Tommy Kirk) and Nancy (Charla Doherty) are having an intimate moment when Nancy's little brother Genius (Ron Howard) interrupts them, showing a concoction that he has made with non-compatible chemicals. Strange enough, the mixture has not blown up. Nancy professes to not being impressed, and Genius returns to his basement laboratory, where he puts an electrical charge to the concoction, resulting in an explosion.
Mike and Nancy quickly enter the lab, and find the concoction has turned pinkish, growing out of a tulip glass that Genius put it in. As they attempt to clean up the basement, a stray cat enters the lab and begins to eat some of the concoction. Mike shoos the cat away, but soon, the three find that the cat has grown enormous! Genius' dog named Wolf chases the cat out of the house, before Mike wonders what they have on their hands. Genius explains the concoction is probably "Ammonia-actride-sulfate-deethyl-chlorazine," but opts to just call it "goo."
Mike takes some, and the three go outside. Feeding it to a pair of white ducks, they watch as the ducks grow large, with Mike deciding that they can easily make a million dollars from Genius' invention. After going back inside, they find that Wolf has also eaten some of the goo, and grown large. The amusement soon turns to shock when Nancy looks out the window and sees the giant ducks are gone.
Meanwhile, the delinquent teens have broken into the closed Hainesville theater. Having cleaned up, they head off to the local go-go dance club. No sooner have they arrived, than the giant ducks appear, amusing everyone as they dance to the music. Mike and Nancy are not far behind, and when Mike claims the ducks are his, one of the delinquent teens named Harry (Robert Random) demands to know how they got so big. Mike just hints that it's a food additive, and a 'million dollar secret.'
Harry tells the other delinquents, who formulate a plan to try and find out more about the food additive. The leader of the delinquents named Fred (Beau Bridges) attempts to woo Nancy, while one of the girls named Jean (Tisha Sterling) attempts to get the information from Mike. However, neither Mike nor Nancy divulge the information Fred or Jean ask for.
The next day, the giant ducks are killed and barbecued at a party in a nearby park. As Mike and Nancy serve up the giant ducks, Genius arrives, intent to show them that he's made more goo...only to watch as the substance dissolves into nothingness. One of the delinquent teen girls named Elsa (Gail Gilmore) notices this, and persuades Genius to take a walk with her, as she hopes he'll tell her where the rest of the goo is.
Later on that evening, the delinquents sneak over to Nancy's house, and one of the teens named Pete (Tim Rooney) sneaks in and steals the substance, but not before a burglar alarm is tripped. Mike and his friends show up, and a brawl ensues, but the delinquents get away with the goo.
Returning to the town theater, some of the teenagers want to use the goo and get rich, when a couple of the guys dare the others to take it. Several refuse to be considered cowards, and everyone takes the goo, causing the teenagers to grow 30 ft tall, and bursting out of their clothes. Some of the girls are ashamed of their size, but some of the guys see this as a way to get back at the adults.
Fashioning clothing from curtains and cloth in the theater, the giants leave and appear at the local teen party, and proceed to dance. One of the girls named Merrie (Joy Harmon) picks up Mike friend Horsey (Johnny Crawford), and holds him to her chest as if he were dancing with her. Mike demands Fred tell Merrie to put his friend down, but when Fred refuses, Mike breaks a wooden chair on Fred's lower leg, causing the dancing to end.
After brushing Mike aside Fred asserts that they are going to take over the town, but his plans fall short when the local Sheriff (Joe Turkel) arrives, and orders the giants to return to the theater where they've been staying.
The Sheriff and Mike show up at the theater the next day, with the Sheriff demanding the giant teenagers leave Hainesville. However, Rick reveals that they have kidnapped the Sheriff's daughter, and use her as leverage against the Sheriff. The Sheriff then gives into the giants demands and surrenders all the guns in the town to them, as well as has food brought to them.
Mike decides to do something about this, and feels that they need to capture one of the giants as their own hostage. Luring Fred into a neighborhood, Mike, Horsey and their friends attempt to capture Fred using their cars and some rope. However, the plan backfires when Pete captures Nancy, and Mike and the others are forced to let Fred go, as the two giants take her to the theater.
Mike then comes up with a new scheme, with Horsey's girlfriend Red (Toni Basil) sent in to distract the male giants with a dance. After this, Mike appears outside the theater with a slingshot, meant as a distraction. Fred goes after Mike with a spear, and sends Merrie back into the theater to watch Nancy and the Sheriff's daughter. While she watches, Horsey shocks her with a large amount of ether-filled cotton, causing her to pass out. This allows Horsey and his friends to free Nancy and the Sheriff's daughter, and recover the guns.
Meanwhile, Genius has failed to make more goo, but has come up with a yellow gaseous antidote that proves itself when Wolf shrinks back to normal size.
Genius quickly takes the antidote and heads to the town square, where he uses it on the giants, shrinking them back down to normal size. Clutching onto their oversized garments, the now shrunken teens run out of town back to their car at the landslide. Realizing the car is damaged beyond repair, they realize they have a long walk ahead of them.
Before they start, they are greeted by several little people who claim they are heading to Hainesville to find out about the goo. After the little people have passed them, the once giant teenagers walk off away from the small village and their car.