- “I'll tell you at their size, that backyard is like 10 miles. Giant blades of grass, huge insects; it's a jungle out there!”
- ―Wayne Szalinski[src]
Having problems at home with his wife Diane (Marcia Strassman), struggling inventor Wayne Szalinski (Rick Moranis) has created an electro-magnetic shrink ray - unfortunately it only seems to make things explode. While at work, his neighbor's son, Ron Thompson (Jared Rushton), hits a baseball into the Szalinskis' attic and starts the shrink ray.
Ron and his older brother Russ Jr. (Thomas Wilson Brown) and Wayne's children, Amy (Amy O'Neill) and Nick (Robert Oliveri), go up to the attic to retrieve the baseball. Luckily, it got stuck in the machine and blocked one of the laser components, resulting in it only releasing enough energy to shrink things instead of blowing them up, and the kids, a nearby couch, and chair are shrunk by the ray when it fires.
As Wayne comes home from a presentation at the lab, he notices his children and his "thinking couch" missing. Without an explanation for this occurrence, he begins to get angry at the machine for being joked about during work. He begins to destroy it as the kids, now 1/4 of an inch tall, try to get his attention, but to no avail. He begins to sweep up the mess and accidentally sweeps them into the trash.
The kids escape the trash bag and find themselves at the far end of the backyard. Knowing that crossing it to get back to the house will take hours at their size, they begin to climb a flower to see how far it is. While clinging to the stem, Nick and Russ Jr. are attacked by a bee. They cling to it for dear life and are separated from Ron and Amy.
Russ Thompson, Sr. (Matt Frewer), Ron and Russ Jr.'s bumbling father, is getting aggravated at his missing sons since he is planning a fishing trip with the family. Russ Jr., who secretly likes Amy, often feels small compared to his dad's standards since he was cut from the football team (although it's revealed later on that he actually quit). While looking at the attic floor, Wayne finds his "thinking couch" in a miniature form. He soon concludes that he has shrunken the kids. Diane comes home and he tells her this. She becomes infuriated with him and tries to help search for the kids.
Meanwhile, Wayne accidentally turns on the sprinklers while searching for the kids in the backyard. Giant splashes of water soon surround them. Amy begins to drown in the muddy water near the flagstone while Wayne turns them off. Russ Jr. saves her before she dies by giving her CPR. The kids continue their journey. On their way, they find a cream cookie in the yard. They meet an ant who befriends them, nicknamed "Antie" by Ron. Wayne and Diane decide to tell Russ and his wife Mae (Kristine Sutherland) about the shrinking of their sons.
They are not pleased, but Mae seems to understand more. At the end of the night, the children decide to sleep in one of Nick's Legos. During the night Amy and Russ Jr. admit their feelings for each other and share a kiss, but are interrupted by an attacking scorpion. Antie, trying to rescue them is fatally wounded by its stinger, though the kids scare it off by bombarding it with stones and sharp sticks (Ron is able to put out two of its eyes). They comfort Antie, who in moments dies from the venom.
In the morning, Tommy, Nick's friend, comes to mow the lawn. The children hear the lawn mower and descend into a wormhole to escape. Wayne and Diane rush outside to make Tommy stop, but unfortunately he does just as the lawn mower is over the hole. It sucks Nick out of hole, but the others hold on to him. They are not killed, but do not get the attention of their parents. They realize that their voices are too high and quiet to be heard by them, but that they can be heard by their dog, Quark. They hold onto his fur and ride him back into the house.
While Wayne is eating a bowl of Cheerios, Nick accidentally lands in it. The others try to get Wayne's attention as Nick is about to be eaten. Quark bites Wayne, who freezes with the spoon halfway to his mouth. Looking through a magnifying glass, he sees Nick. The kids communicate to their parents that the baseball went through the window and started the machine. Armed with this information, Wayne pieces together how the accident made it work and brings the kids back to normal size after using it to shrink and regrow Russ.
A few months later, during Thanksgiving, the two families eat an enlarged turkey and are happy. Russ has come to terms with Russ Jr. quitting football, Wayne and Diane patch up their problems, Nick and Ron finally become friends, and Russ Jr. and Amy begin dating. The movie ends with Quark eating a giant bone as a Thanksgiving treat. But as it begins to black out, it reopens suddenly to Nick, realizing a joke that Russ Jr. made earlier about learning CPR in French class. It closes with him laughing, getting the joke.
- Rick Moranis as Wayne Szalinski
- Marcia Strassman as Diane Szalinski
- Amy O'Neill as Amy Szalinski
- Robert Oliveri as Nick Szalinski
- Matt Frewer as Russ Thompson, Sr.
- Kristine Sutherland as Mae Thompson
- Thomas Wilson Brown as Russ Thompson, Jr.
- Jared Rushton as Ron Thompson
- Carl Steven as Tommy Pervis
- Mark L. Taylor as Don Forrester
- Kimmy Richardson as Gloria Forrester
- Lou Cutell as Dr. Brainard
- Laura Waterbury as Female Cop
- Trevor Galtress as Male Cop
- Martin Aylett as Harold Boorstein
- Janet Sunderland as Lauren Boorstein
- Craig Richard Nelson as Professor Frederickson
- Frank Welker as Antie (voice)
The film has earned a 75% "fresh" rating at Rotten Tomatoes, with generally positive reviews. Roger Ebert, of the Chicago Sun Times, gave a negative review, stating: "The special effects are all there, nicely in place, and the production values are sound, but the movie is dead in the water." Caryn James, of The New York Times, gave a positive review, saying: "As sweet, funny, and straightforward as its title." Variety gave another positive review stating, "[It's] in the best tradition of Disney -- and even better than that, because it is not so juvenile that adults won't be thoroughly entertained."