Based on Hank Ketcham's popular comic-strip, Dennis the Menace became a staple in countless households throughout the mid-80s. With the help of his precocious pals and his shaggy dog Ruff, Dennis uses his curiosity and imagination to keep his pleasant Midwestern neighborhood in a constant state of confusion and chaos.
An alien from another planet lands in Dennis room in a quest for peace and quiet... but Dennis wants to show him around! A mad romp ensues with Dennis taking the alien to Mr. Wilson's house. When Wilson calls the police he is thought to be a quack and carted away. Dennis and his family go on a trip on a train, however Dennis forgets his suitcase. Mr. Wilson, trying his best to make sure Dennis leaves, runs onto the train to give him the suitcase... but, he cannot get back off and is now stuck with Dennis on the train! While there, Dennis sees a man with a trenchcoat and a briefcase and figures him to be a spy. With Mr. Wilson unwillingly helping out, Dennis proves the man to be a spy. Dennis and Margaret both find a bottle from which a Genie emerges. They argue over who gets the wishes, but agree on one wish each. Margaret uses her wish to make her doll come to life, which gets her into jeopardy, and Dennis uses up his own wish to save her.
Mr. Wilson is painting his house and Dennis invites himself over to help out. Needless to say, Dennis' "helping" gets Mr. Wilson close to a nervous breakdown. Dennis accompanies Henry to an auto showroom, where Dennis harasses the poor salesman so, that Henry buys a new car almost to avoid a law-suit. He discovers that the purchase entitles him to watch the actual car being built on the assembly line, with the hapless salesman acting as their guide. Now Dennis turns his impulsiveness toward the robot assembly line, screwing it up completely, and their "new" car comes out looking like futuristic junk!. A Raymond Chandler-type takeoff in which Dennis pretends to be a hard-boiled private eye, in the Bogart tradition, tracking down his missing pooch, Ruff, by questioning his friends and neighbors.
Mr. Wilson slips on Dennis' skateboard and has to go to the hospital. Once there, Dennis arrives to cheer Wilson up... Wilson is in for the time of his life! The kids discover an abandoned old gothic house that Dennis decides would make a great club-house. The kids begin cleaning the place up until a pair of ghosts appear! The others want to leave the place, but Dennis who never once sees the ghosts chides them for being scaredy cats, and gets them to stay. Finally the "ghosts" are revealed as a couple of crooks who are using the old house to hide their ill-gotten gold, and using "scare tactics" to frighten the kids away. Dennis saves Margaret's life, and she won't let him forget it. Desperate to get Margaret out of his hair, he tries to arrange to have the girl save his life. All his efforts are futile...until an escaped grizzly bear happens on the scene.