Eccentric inventor Wayne Szalinski is obsessed with perfecting his futuristic electromagnetic shrinking machine, much to the frustration of his long-suffering wife, Diane, and their two teenagers, Amy and Nick. When Diane demands a trial separation, Wayne remains locked in his attic laboratory, determined to make the contraption work. Meanwhile, next-door neighbors,
the Thompsons, prepare for a weekend fishing trip, led by the pragmatic Russ and his baseball-obsessed younger son, Ron. Tension between the two households runs high, fueled by Wayne’s erratic experiments causing literal neighborhood vibrations.While attempting to retrieve a rogue baseball from Wayne’s attic, Ron and his older brother Russ Jr. are joined by Amy and Nick. Fate intervenes when Wayne's malfunctioning machine finally fires, reducing the four teenagers—along with the entire room's debris—to a mere quarter-inch in height. Unaware of the catastrophe, Wayne returns to his lab, discovers the machine still isn't working properly, and in a fit of rage, smashes the device and sweeps the microscopic youths into a garbage bag, unknowingly tossing them out into the overgrown backyard jungle.Regaining
consciousness, the kids realize their terrifying new reality. Their journey across the perilous lawn becomes an epic survival saga. They navigate towering blades of grass like a dense jungle, face a torrential lawn-sprinkler rainstorm that nearly drowns them, and encounter terrifying wildlife. A friendly ant, which they name Antie, becomes a loyal companion, carrying their burdens and protecting them from predators like a giant scorpion and a voracious bee. During the harrowing trek, bonds form; Nick’s intellect proves invaluable, Russ Jr. steps up as a protector, and romantic sparks ignite between Russ and Amy.Back in the house, Wayne and Diane reconcile when Diane returns early. Discovering the missing kids and realizing what his machine actually did, Wayne desperately recreates his lab setup. In the backyard, tragedy strikes when Antie sacrifices its life to save the children from a monstrous spider, leaving the kids heartbroken and vulnerable.Inside, Wayne drops his last remaining particle-expansion pod onto the carpet. Sweeping the floor frantically, he spots the kids riding a runaway remote-controlled car. Wayne manages to vacuum them up just in time, placing them inside the restored machine. With a blinding flash of light, the kids are successfully restored to their normal size. The ordeal transforms both families: the Thompsons and Szalinskis become close, loving friends, and Wayne achieves the ultimate validation when his shrinking invention finally works.