Plot
Five-year-old Dennis Mitchell is a constant source of mischief, especially to his retired next door neighbor, George Wilson. One day, George pretends to be asleep to avoid Dennis, who mistakes this for illness and shoots an aspirin into his mouth with a slingshot. Dennis's parents, Henry and Alice, try to discipline him as they get ready for work and leave him with his best friend Joey McDonald at the home of their unloved classmate, Margaret Wade. As they fix up an abandoned treehouse in the woods, itinerant thief Switchblade Sam arrives in town.
Vacuuming spilled paint in the garage, Dennis inadvertently shoots a glob of it onto George's barbecue grill; tasting it, he suspects Dennis. Henry and Alice leave Dennis with teenage babysitter Polly, who invites her boyfriend, Mickey. Sneaking outside, Dennis pranks them by ringing the doorbell and hiding until Mickey tapes a thumbtack to it. George investigates the vacuum in the Mitchells' garage and accidentally shoots himself in the gonads with a golf ball. Hoping to confront the Mitchells, George pricks his thumb on the tack. Mistaking him for the prankster, Polly and Mickey douse George with water and flour.
Dennis plays with George's dentures, loses the two front incisors and replaces them with Chiclets before George has his picture taken for the local newspaper. Henry and Alice need to leave on respective business trips but are unable to find anyone willing to babysit Dennis, except Martha, George's wife. George is infuriated that Dennis has replaced his nasal spray with mouthwash and the latter with toilet cleanser. Dennis lets his dog Ruff in the Wilsons' house, leading George to mistake him for Martha. In the attic, Dennis's carelessness causes George to slip on mothballs. Meanwhile, Sam commits a string of burglaries in town and is noticed by Chief Bennett, who tells him to leave the area or be arrested.
George has been chosen to host his garden club's "Summer Floraganza", having spent almost 40 years growing a rare orchid that will finally bloom that night. During the party, Dennis presses the garage door button and it opens, upending the entire dessert table. While the Wilsons and their guests await the flower's nocturnal display, Sam breaks into the Wilsons' house and steals George's antique coin collection. Dennis, who heard Sam and discovers the empty safe, alerts George, distracting everyone from the flower's brief blooming, and it dies soon after. Furious and unaware he has been robbed, George coldly chastises Dennis, who flees to the woods and is caught by Sam. Henry and Alice arrive home to learn Dennis has disappeared, prompting a town-wide search. Even a guilt-ridden George joins the search after realizing Dennis was telling the truth about the burglary.
Sam prepares to leave town with Dennis as an unsuspecting hostage. Showing him the proper way to tie him up, Dennis handcuffs Sam, loses the key, unintentionally bludgeons him and sets him on fire. Just as Dennis discovers George's stolen coins and realizes Sam is a burglar, the latter attempts to stab Dennis but is snared in a rope caught by a passing train.
The next day, Dennis returns home with Sam and George's recovered coins. Chief Bennett arrives and alleges to know Sam was a criminal all along. While Sam is being arrested, Dennis's friends show up and are happy Dennis is safe and sound. Dennis returns the knife to Sam but Chief Bennett slams the patrol car door on Sam's hand, causing him to lose the knife. The Mitchells and Wilsons make amends and Alice mentions she can bring Dennis to work with her because her office now has a daycare center. George and Martha insist they would be happy to watch Dennis, just as Dennis accidentally flings a flaming marshmallow onto George's forehead.
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