Five-year-old Dennis Mitchell is a constant source of mischief, especially to his retired next door neighbor, George Wilson. George pretends to be asleep to avoid Dennis, who mistakes this for illness and shoots an aspirin into his mouth with a slingshot. Dennis' parents, Henry and Alice, try to discipline him as they get ready for work, and leave him with his friend, Joey McDonald, at the home of their unloved classmate, Margaret Wade. As they fix up an abandoned treehouse in the woods, itinerant bandit Switchblade Sam arrives in town. Vacuuming up 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 a teenage babysitter named Polly, who invites over 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, he pricks his thumb on the tack; mistaking him for the prankster, Polly and Mickey douse him with water and flour. Switchblade Sam commits a string of robberies throughout town, and is noticed by Chief Bennett. Dennis plays with George's dentures, loses the two front incisors, and replaces them with
Chiclets just before he has his picture taken for the local newspaper. Henry and Alice leave on business trips, but are unable to find anyone willing to babysit Dennis besides 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' carelessness causes George to slip on mothballs. George has been chosen to host his garden club's "Summer Floraganza", having spent almost forty years growing a rare orchid that will finally bloom that night. During the party, Dennis presses the garage door button, and it opens and upends the entire dessert table. While the Wilsons and their guests await the flower’s nocturnal display, Switchblade Sam robs their house, stealing George’s antique coin collection. Dennis, who heard him and then discovers the empty safe, alerts George, distracting everyone from the flower's brief blooming, and it dies afterward. Furious and unaware that he has been robbed, George coldly chastises Dennis, who flees to the woods and is caught by Switchblade Sam. Henry and Alice arrive home to learn he has disappeared, prompting a town-wide search, and even a guilt-ridden George sets out to look for him after realizing that he was telling the truth about the robbery. Switchblade Sam prepares to leave town with Dennis as an unsuspecting hostage. Showing him the proper way to tie him up, Dennis handcuffs him, loses the key, unintentionally bludgeons him, and sets him on fire. Just as Dennis discovers George’s stolen coins and realizes Switchblade Sam is a thief, the latter attempts to stab him but is snared in a rope caught by a passing train. The next morning, Dennis returns home with the captured Switchblade Sam and George's recovered coins, to the relief of George and the entire neighborhood. Switchblade Sam is arrested, and Dennis naïvely returns his knife and he attempts to stab him with it again, but Chief Bennett closes the police car door on his hand, causing him to drop the knife down a storm drain and wince in pain before being driven away. Dennis and George make amends, and Alice mentions that she can bring Dennis to work with her as her office now has a day care center. George insists he would be happy to watch Dennis himself, just as Dennis accidentally flings a flaming marshmallow onto his forehead. During the closing credits, Dennis gets Alice's condescending coworker, Andrea, caught in the office copy machine. ==Cast==