Dennis Mitchell is a five-year-old boy who lives with his parents Henry and Alice and is the bane of next door neighbor George Wilson's existence. Because of his trouble-making but unintentional behavior, his parents often struggle to find suitable babysitters to deal with him. On one night, they manage to get one named Polly and her boyfriend Mickey to babysit him, but repeated doorbell pranks from Dennis push the two too far (not knowing he is behind this), and they end up pulling a prank on George when he rings the doorbell to scold Dennis after finding paint and wood in his food in an earlier incident. While all of this and the rest of the events in town go on, a burglar named Switchblade Sam arrives in town and begins robbing houses, as well as striking fear into the children that he meets.Dennis' parents are both called away on business trips at the same time, and when everyone they know refuses to look after him, they turn to George and his wife Martha (who loves Dennis and sees him as a surrogate grandson) to look after him. George is further irritated by him spilling bath water on the bathroom floor, swapping chemicals, and bringing his pet dog, Ruff, into the house for a while. All of this is happening around the time the Summer Floraganza, a long-awaited event, is scheduled to happen. As a longtime member of the local garden club, George is chosen to host it. He is excited to have this honor, as he has been growing and nurturing a rare plant for forty years. After growing for the said length of time, its flower finally blooms, only to die several seconds later.Alice gets stuck at the airport due to a storm, thus forcing Dennis to stay with the Wilsons for an extra night, which coincides with the unveiling of the plant and its blooming to the members of the garden club. While that is happening, he is sent away for causing trouble (namely overturning the dessert table when he pushed a black button, which he found out opened the garage door), when he hears Switchblade Sam robbing the house. He goes downstairs and finds George's gold coins missing. He runs outside to tell him just as the flower is beginning to bloom, which causes him and all the guests to miss its entire lifespan of the blooming flower. Not knowing about the robbery, he severely scolds him, tells him that he has no use for him, and that he doesn't want to know or see him anymore. Shortly thereafter, Dennis gets on his bicycle (with a wagon attached) and rides off into the night, eventually bumping into Switchblade Sam in the woods. Sam then abducts him, intending to use the child as a hostage.Dennis' parents return home and learn of his departure, and they, the authorities, his friends (Joey, Margaret, and all the neighborhood kids), and George (who feels intense guilt and remorse after remembering all the things he said to him and having now discovered his house was burglarized and that Dennis actually had good intentions when he tried to tell him) search all night for him. Around the same time, Dennis unintentionally but effectively defeats Switchblade Sam by tying him up and handcuffing him, losing the key, and repeatedly setting him on fire, among other things.He returns to George's house the next morning with Switchblade Sam in his wagon, having also recovered George's gold coins, and Sam is taken into police custody by a slightly amused sheriff who had previously advised him to leave town. Dennis and George make up, and the Mitchells and Wilsons become friends on better terms. That night, George explains that he's learned some things about kids: kids are kids, and that one has to play by their rules, roll with the punches, and expect the unexpected, in which afterward Dennis, trying to get a flame on a marshmallow to go out, then accidentally hits him in the forehead with it.The film's end credits are accompanied with Dennis inadvertently humiliating his mother's egotistical coworker, Andrea, while she is using a photocopier. Dennis impishly whacks the "PRINT" button and runs away, with other workers looking on. Andrea loses her balance, and her head gets pinned face-down on the scanner-bed, and the machine relentlessly flashes its blinding light in her eyes as it repeatedly "takes her picture" and spews out page after page of black-and-white "photos" showing her various agonized facial expressions as she squirms about on the scanner bed.