While retrieving the newspaper, George Wilson’s quiet morning is disturbed by the oncoming clanking of his rambunctious neighbor child, five-year-old Dennis Mitchell, riding his tricycle. As the boy wanders into his house, Mr. Wilson hides by jumping into bed and pretending to be asleep. Assuming he is ill, Dennis shoots an aspirin tablet into his mouth with a slingshot, causing him to choke. Frightened, Dennis returns home to his parents, Henry and Alice, who telephone Mr. Wilson to apologize. Despite his anger, Mr. Wilson insists he is not a “grump” for requesting the Mitchells reprimand their son. Later, Mr. Wilson goes to his backyard to admire the rare orchid he intends to show off at his garden club’s upcoming Summer Floraganza event. With Alice returning to work, Dennis is forced to spend his summer afternoons at the house of Margaret Wade, a “bossy” girl he greatly dislikes. Along with Dennis’s friend, Joey, the children pass time repairing a tree house in the forest. Meanwhile, a vagrant robber known as “Switchblade Sam” jumps off a train, looks over the town, and terrorizes several children. One afternoon, Dennis reaches for his confiscated slingshot in the garage and knocks a can of paint off a high shelf. Using a vacuum to clean the mess, Dennis accidentally reverses the suction and spits a blob of paint into Mr. Wilson’s barbeque grill. When his dinner tastes strangely, Mr. Wilson is convinced that Dennis is responsible and sneaks into the Mitchells’ garage to investigate. Inside, Dennis’s babysitters grow frustrated by the boy’s repeated doorbell pranks and booby-trap the front stoop, but inadvertently douse Mr. Wilson in water and baking flour when he approaches the house. The next morning, Mr. Wilson prepares to have his photograph taken in his garden, but discovers Dennis has replaced the front teeth of his dentures with two pieces of Chiclets chewing gum. At the park, Switchblade Sam steals a woman’s purse. He is nearly caught by a police officer, who questions his grungy appearance and advises him to leave town. When Alice and Henry are required to take simultaneous business trips, the couple struggle to find a babysitter and resort to asking George and Martha Wilson. One rainy afternoon, Dennis pesters Mr. Wilson with questions about his rare coin collection, replaces his nasal spray with mouthwash, and substitutes the mouthwash with bathroom cleaner. As Mr. Wilson goes to bed, Martha expresses her regret about not having children, and is disappointed by her husband’s gruff lack of emotion. Deciding to apologize, Mr. Wilson goes downstairs and unknowingly kisses a dog Dennis let into the house instead of his wife. The morning of the Summer Floraganza, Martha prepares to send Dennis home, but learns that Alice’s flight has been canceled. When the event begins, Dennis charms the Wilsons’ guests, but accidentally upends the dessert table and is sent to his room. While the attendees await the singular moonlight blossoming of Mr. Wilson’s prized orchid, Switchblade Sam sneaks into the house and steals Mr. Wilson’s coin collection. Dennis announces the robbery, distracting the guests just as the flower opens. Infuriated, Mr. Wilson yells at Dennis, causing the boy to run away. When Alice returns home a few hours later, Mr. Wilson guiltily remembers all the times he scolded Dennis and leaves to find him. Meanwhile, Dennis is taken hostage by Switchblade Sam. They set up camp under a bridge along the river, but the criminal quickly grows frustrated with Dennis’s incessant talking. When Dennis accidentally shoots the robber in the head with his slingshot and sets his pants on fire, Switchblade Sam attempts to constrain the boy. Criticizing his knot-tying skills, Dennis offers to demonstrate, and successfully binds the criminal’s wrists and ankles. As Switchblade Sam struggles to cut himself free, he is knocked unconscious by Dennis, who drops a piece of firewood from the bridge. Dennis then covers the criminal with a blanket, which catches flame in the campfire. Upon recovering Mr. Wilson’s coins, Dennis drags Switchblade Sam home in his wagon. Overjoyed by Dennis’s return, Mr. Wilson embraces the boy, and Switchblade Sam is arrested. When Alice announces she intends to send her son to day care, Mr. Wilson has a change of heart and insists he and Martha watch over him. Just as he makes this declaration, however, Dennis tries to extinguish a roasted marshmallow, which flies across the yard and lands on Mr. Wilson’s forehead.