In the wake of their previous adventures, inventor Wayne Szalinski and his family are starting anew in the vibrant state of Nevada, where they joyfully welcome their energetic two-year-old son, Adam. With Wayne's wife, Diane, leaving to help their daughter Amy transition to college life, the household becomes a lively hub for Wayne, their teenage son Nick, and their curious little one, Adam.
As Nick begins to develop a budding crush on Mandy Park, who Wayne has enlisted to babysit Adam, the family dynamic quickly shifts. Wayne’s passion for his inventions leads him to [Sterling Labs], where he has meticulously created an enhanced version of his shrink ray, now capable of enlarging objects. In a flawed yet exhilarating experiment, Wayne, along with Nick and Adam, attempts to test the device on Adam’s cherished toy, Big Bunny. Yet as chaos unfolds, little Adam, ever the mischievous toddler, inadvertently activates the device, initiating a series of surreal events.
In a dramatic turn, electrical surges from the microwave amplify Adam's beloved toy, causing both Adam and the Big Bunny to grow uncontrollably to heights they never could have imagined—Adam now towers at an astounding 7 feet. As Wayne and Nick attempt to process this bizarre transformation, they are deterred by the scheming Dr. Charles Hendrickson, a jealous colleague who aims to exploit Wayne’s groundbreaking invention for his own gain.
Upon Diane's sudden return, she finds her family entangled in this extraordinary predicament, struggling alongside Wayne and Nick to manage their enlarged son. In a frantic effort to restore order, they set out to retrieve Wayne's original shrink ray from a storage unit. However, their plan encounters further complications when Mandy enters the scene, panics, and collapses. Amid confusion, Nick must carefully explain their bizarre reality to her, just as Adam, now bombarded with television electrical waves, unexpectedly grows even larger, bursting through a wall in the process.
As Nick and Mandy race against time to find Adam, they unexpectedly get caught and taken away. Meanwhile, Wayne and Diane return home only to face Dr. Hendrickson and a stoic U.S. Marshal, who is keen to see Wayne removed from his position, intending to imprison him and Diane while subjecting Adam to sinister experiments. Yet, when it appears all hope is lost, Clifford Sterling, the company's chairman, arrives, not only defending Wayne but also dismissing Dr. Hendrickson for his devious plans.
Unbeknownst to the family, Adam finds himself in the midst of a powerful electrical storm, rapidly growing to a staggering 50 feet. Mistaking Nick and Mandy for toys, he playfully scoops them into his overalls pocket before heading toward Las Vegas, trailed closely by his frantic parents and pursuing authorities.
As the chaos progresses, Wayne and Sterling uncover a crucial truth—that the dazzling neon lights of Las Vegas will only exacerbate Adam's size increase. Meanwhile, Dr. Hendrickson, in cahoots with the ruthless board director Terrence Wheeler, collaborates on a scheme to dethrone Sterling.
In a desperate move to recapture Adam, Dr. Hendrickson commandeers a military helicopter in an attempt to subdue the giant boy. Before he can execute his plan, Wayne resolves to return Adam to his original size using his faithful shrinking ray. But with Adam's towering stature, the task proves daunting, as he grows even more—ballooning to 112 feet tall.
As chaos grips Fremont Street in Las Vegas, Dr. Hendrickson finds himself firing tranquilizer darts from his helicopter, provoking Adam’s cries among a crowd of sympathetic witnesses. In a heartwarming moment, Diane, driven by maternal love, steps in to console her enormous son, ultimately calming him long enough for Wayne to use his shrinking device.
As normalcy triumphs, both Adam and Diane return to their original sizes. Dr. Hendrickson, humbled by the events, begrudgingly congratulates Wayne, only to be swiftly knocked out by a punch from Diane.
In a surprising twist, Wayne and Diane learn that Nick and Mandy have been shrunk down and are still nestled in Adam's pocket, now surprisingly enamored with each other. With the authorities closing in, Wayne grants them a fleeting moment of privacy before restoring them to their normal proportions. As they cherish each other's company, a Nevada State Police officer appears, leading Adam to take control of his much-loved Big Bunny, now 112 feet tall, reveling in innocent fun that brings a touch of joy amidst the pandemonium.
