In a world where scientific zeal meets chaos, the eccentric inventor Wayne Szalinski is utterly absorbed in his groundbreaking project—a ray gun that can shrink and enlarge objects. His fixation troubles his pragmatic wife Diane and their two children, 15-year-old Amy and 12-year-old Nick, who are more focused on their daily lives than their father's unusual obsession. As fate would have it, the Szalinski family's next-door neighbors, Big Russ Thompson, Mae, and their son Ron are gearing up for a fishing excursion, while their older son, Little Russ, expresses his reluctance about the trip.
Disaster strikes when Wayne leaves for a conference and young Ron inadvertently activates the invention after hitting a baseball through the attic window. The result? A sudden shrinkage of Ron, Nick, Amy, and ultimately Little Russ, leaving them desperately trying to catch their parents' attention—though only their loyal dog, Quark, can hear their tiny cries.
Upon returning home, Wayne is confronted with an array of peculiar happenings, including a broken window and frantic shouts that seem to go unheard. Mere frustration intensifies as Wayne, already annoyed by a botched presentation, inadvertently sweeps up the microscopic kids along with debris into a trash bag. They just manage to escape capture, found initially in a dustpan.
As concern mounts regarding the missing children, Mae and Big Russ abandon their fishing plans and alert the authorities. Meanwhile, Wayne starts to realize what has occurred but his attempts to correct the mess are thwarted by his own klutziness. A string of misfortunate events leads to Amy nearly drowning in a muddy puddle, which prompts Little Russ to perform a heroic CPR rescue.
The drama escalates further as Wayne confides in Diane about the incredible calamity caused by the ray gun, urging her to help locate their children. However, trying to convince the Thompsons of the bizarre truth proves to be a challenge, as they remain doubtful about Wayne's claims. With tension brewing and the children still missing, the situation takes a wild turn as night falls, and the kids encounter one of Nick's forgotten Oatmeal Creme Pie cookies. The crumbs lead them to craft an impromptu device to capture a friendly little ant they name "Antie." With newfound courage, they ride Antie towards safety, taking refuge within a sturdy Lego brick as darkness envelops them.
Beneath the starlit sky, the budding relationship between Amy and Little Russ unfurls into a sweet moment as they share a kiss, only to be abruptly interrupted by the terrifying appearance of a scorpion. In a display of bravery, Antie makes the ultimate sacrifice, fighting off the scorpion so the children can escape unharmed.
As dawn breaks, Tommy, Nick's friend, arrives with a lawnmower that threatens their tiny universe. Panic ensues, and the kids hide in an earthworm tunnel as Tommy begins mowing. Just as the chaos peaks, Wayne and Diane swoop in, halting the mower from causing calamity. With a sigh of relief, the children mount Quark to flee towards safety, while Nick narrowly avoids falling into a bowl of Cheerios, escaping his father's breakfast plans.
Quark's antics eventually lead Wayne to notice the children's precarious situation. The families gather in the attic, where they employ a game of charades to communicate vital information about the mishap. They discover that the baseball lodged in the laser was responsible for the earlier overloads and explosions.
With a renewed sense of purpose, Wayne acts swiftly to resolve the issue, and Big Russ steps forward as the first volunteer for the groundbreaking correction of the system. With a deep breath, he is restored to his original size, closely accompanied by the children. The air celebrates as the Szalinskis and Thompsons toast to their new friendship, forged amidst adversity.
In a heartwarming final scene, months later, the two families come together on Thanksgiving Day, united around a massive turkey, while Quark indulges in a giant dog biscuit. They give thanks for the extraordinary experiences that brought them closer together in a truly memorable journey.
