Dennis Mitchell, a spirited five-year-old boy, resides with his parents, Henry and Alice. However, he happens to be a constant source of frustration for his next-door neighbor, George Wilson. Due to his mischievous, albeit unintentional antics, finding a suitable babysitter for Dennis becomes a challenging task for his parents. On one particular night, they secure a babysitter named Polly and her boyfriend, Mickey. Unfortunately, their evening takes a chaotic turn as Dennis’s incessant doorbell pranks push the couple to their limits. In a moment of frustration, they pull a prank on George when he comes to confront Dennis after discovering paint and wood in his food from a previous mishap.
Amidst this chaos, a notorious burglar known as Switchblade Sam descends upon the neighborhood, instilling fear in local children while orchestrating a series of home robberies. As fate would have it, both of Dennis's parents are called away for business trips simultaneously, leaving them no choice but to enlist George and his wife, Martha, to take care of Dennis. Though Martha adores Dennis, seeing him as a surrogate grandson, George is less than thrilled as he deals with the aftermath of Dennis's antics, including water spills, swapped chemicals, and the unexpected company of Dennis's pet dog, Ruff.
This all unfolds as the town eagerly anticipates the Summer Floraganza—a much-cherished event. George, a dedicated member of the local garden club, is selected to host this significant occasion. He is overjoyed, especially since he has nurtured a rare plant for an astounding forty years, and on this momentous day, its flower finally blooms, only to wilt mere seconds later.
Complications arise when a storm stranding Alice at the airport forces Dennis to stay with the Wilsons for an extra night, coinciding with the garden club's much-anticipated unveiling of George's precious plant. During the event, Dennis's mischief reaches a peak when he inadvertently causes mayhem by pushing a black button that unexpectedly opens the garage door, leading to further trouble. It is at this moment that he overhears Switchblade Sam robbing the house. Dennis races downstairs to find George's prized gold coins gone. Eager to inform George just as the flower begins to bloom, he tragically distracts the guests and the host, missing the brief spectacle of the plant altogether. In his frustration, George scolds Dennis harshly, declaring he has no need for him anymore.
Heartbroken, Dennis hops on his bicycle with a wagon in tow and rides off into the night, where he unwittingly crosses paths with Switchblade Sam, who then abducts him with the intent of using him as a hostage. Back at home, Dennis's parents learn of his disappearance and, along with concerned friends and neighbors—including George, who wrestles with remorse over his earlier words—set out to search for him.
In a twist of fate, Dennis manages to outsmart Switchblade Sam through a series of accidental but effective moves, from tying him up to unintentionally setting him ablaze repeatedly. The next morning, Dennis triumphantly returns to George with Sam in his wagon, having also recovered the stolen gold coins, leading to an amused but slightly exasperated sheriff taking Sam into custody.
Reconciliation follows as Dennis and George mend their relationship, and the families of Dennis and George find themselves on friendlier terms. That evening, George reflects on what he's learned: that kids will be kids and that one must adapt to their whimsical world—an insight humorously punctuated when Dennis accidentally bumps him on the forehead while trying to extinguish a flame on a marshmallow.
As the end credits roll, a comical scene unfolds where Dennis inadvertently embarrasses his mother's overbearing coworker, Andrea. With a quick press of the "PRINT" button on the photocopier, he dashes away, leaving behind a flustered Andrea as the machine relentlessly flashes its light and churns out page after page depicting her various dismayed expressions.
