John Hughes continues the trend he began with the Home Alone series in Dennis the Menace, the Hughes-scripted film version of Hank Ketcham’s long-running comic strip, cartoon show, and television comedy. The film opens as Dennis (Mason Gamble) is seen riding down a sidewalk in a beautiful and idealistic suburban town on his training-wheeled bike with his red wagon (filled to the brim) attached – cans on string clattering behind him, baseball cards flapping in the spokes of the wheel and his dog Ruff following him.
“Hey, Mr Wilson!” he calls after slamming on the brakes to his bike in front of the yard belonging to his much put-upon neighbor, Mr. Wilson (Walter Matthau). Half of the film concerns vignettes of small-town Hank Ketcham life as Dennis’ mom Alice (Lea Thompson) starts a new job, Dennis stays over at the house of his friend Margaret Wade (Amy Sakasitz), and Mr. Wilson and his gentle, well-meaning wife, Martha (Joan Plowright), mind Dennis while his parents are out of town. Through all of this, Dennis continually gets into Mr. Wilson’s hair.
Dennis’s parents get stuck in the airport because of 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 elders of the garden club. While that is happening, Dennis is sent up to the guest room for causing trouble (namely pushing the black button that opened the garage door, which overturned the dessert table). While there, he hears Switchblade Sam robbing the house. He goes downstairs and finds George’s gold coins missing. He runs outside to tell George, which causes him to miss the entire lifespan of his flower.
More concerned with that than the robbery, George severely scolds Dennis, 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 tricycle (with the wagon still attached) and heads off into the night. He eventually goes into the park and then runs into Switchblade Sam. Sam then abducts Dennis, intending to use the child as a hostage.
After Dennis’s parents return home and learn of his departure, they search all night for him — along with his friends (Joey, Margaret, Gunther and the neighborhood kids), the authorities and George (who feels intense guilt and remorse after remembering all the things he said to and about Dennis, and also realizes that he had his house burglarized and that Dennis actually had good intentions when he tried to tell him). Meanwhile, Dennis unintentionally, but effectively, defeats Switchblade Sam by tying him up and handcuffing him, losing the keys, and repeatedly setting him on fire.
Dennis then returns to George’s house the next day with a tied-up and handcuffed Switchblade Sam in his wagon, having also recovered George’s gold coins. Meanwhile, Sam is taken into police custody by a slightly amused sheriff (who had previously advised Sam to leave town). Dennis and George make up, and the Mitchells and Wilsons become friends on better terms. That night, George suggests that he and Martha can watch Dennis while Henry and Alice are at work.
However, Alice and Henry, remembering all those times Dennis accidentally messed things up, try to turn his suggestion down gently. “You have to remember back to all those times Dennis upset you,” says Henry.
“Well, this is a new ballgame,” remarks George. “I’ve learned a few things. Kids are kids. You have to play by their rules. Why? If you can’t do that, then you’re headed for trouble. You have to roll with the punches. You have to expect the unexpected.”
Dennis tries to get the fire on a marshmallow to go out by first blowing on it, and then waving it a little. Finally, he accidentally causes it to fly off the stick and then hit George in the forehead with it.
The film’s credits are accompanied with Dennis humiliating his mother’s egotistical co-worker, Andrea, using a photocopier.