- After a failed attempt to find the perfect birthday present for Mr Wilson, the mischievous little terror, Dennis, tries to save him from a pair of swindlers and their bogus youth remedies. Will he manage to keep their friendship strong?
- The day starts out fine. It's Mr. Wilson's birthday and guess who shows up uninvited? Dennis Mitchell and a few of his bug friends. After getting a cake in the face and falling down the stairs in a wagon, Mr. Wilson wishes Dennis had another old man to bother. His prayers are answered when Dennis' grandfather, Mr. Johnson, comes over announcing he's going to move in with them. So once again all is fine until Mr. Wilson finds out he's getting old, and Mr. Johnson can do everything better than him. One day at a carnival, Mr. Wilson meets the Professor and his assistant, Sylvester, who claim to be specialists in making people younger--for a price. It's a scam but Mr. Wilson is too naive to know. The first few attempts at decreasing Mr. Wilson's age fail thanks to Dennis. Mr. Wilson tries to do things better than Mr. Johnson when he follows them to the swimming pool but fails. But it's Dennis who catches the swindlers at their own game when Mr. Wilson wants to sell his house and the Professor and Sylvester show up disguised as plumbers, electricians, realty men, and exterminators and make large holes in Mr. Wilson's house--not to mention his check book, but it's Dennis to the rescue and everyone is happy. Sometime later, Mr. Johnson takes Dennis to the Grand Canyon in his motor home. The mistake is that it is parked awful close to the canyon and its wheels are stopped with a rock, which Dennis collects while rock hunting. Back at home, Mr. and Mrs. Wilson watch Mr. Johnson getting rescued from the canyon and Mr. Wilson proclaims, "He's a menace!"—Anonymous
- Mischievous tyke Dennis is stirring up trouble as usual for his long-suffering neighbor, Mr. Wilson. When Dennis delivers an unwanted birthday surprise that leaves him feeling that much older, he buys a youth potion from the Professor and his assistant, Sylvester. With the bogus potion doing nothing to restore his vigor and being at risk of being swindled out of even more money, it's up to him and Dennis to beat the con artists at their own game.—Anonymous
- After a failed attempt to find the perfect birthday present for his beloved next-door neighbor, Mr. Wilson, the mischievous seven-year-old little terror, Dennis Mitchell, learns that his young-at-heart grandfather, Mr. Johnson, is moving in with them. As a result--exhausted and jealous, Mr. Wilson will fall prey to a conniving pair of swindlers intent on rinsing him for every penny with their endless array of bogus youth remedies, however, not if Dennis can help it. He always had Mr. Wilson's best interests at heart. Will he manage to keep their friendship strong?—Nick Riganas
- Dennis Mitchell (Justin Cooper) is finally back, and worse than ever. Dennis goes over to Mr. Wilson's (Don Rickles) house to offer him one of several gifts for his birthday. These include lizards, snakes, bugs, and other creatures. Inadvertently, the critters escape their jars as Dennis is clumsy. The critters crawl all over Mr. Wilson and scare the wits out of him. The ordeal ends with Mr. Wilson unintentionally riding down a flight of stairs in Dennis' red wagon and accidentally getting his birthday cake thrown in his face by Martha (Betty White).
Soon after this incident, Mr. Johnson, Dennis' grandfather (George Kennedy), shows up and announces that he is moving in with the Mitchells Henry (Dwier Brown) and Alice (Heidi Swedberg). Dennis starts spending more time with him than his annoyed neighbor. Dennis has friends Margaret (Jacqueline Steiger), Joey (Danny Turner) and Gina (Alexa PenaVega). Margaret believes that Dennis and she are destined to be together, but Gina has attracted Dennis with her bugs and other critters. She goes to Mr. George for advise, and Martha tells Margaret to share Dennis's interests. Margaret lures Dennis with fake bugs. But Gina exposes Margaret by grossing her out with a real bug.
Mr. Wilson is very upset that he's getting older and cannot keep up with Mr. Johnson or Dennis. George cannot throw balls, go diving off a swimming pool platform or fix a car, all of which Johnson can do with consummate ease. He gets tricked by two con men named Professor (Brian Doyle-Murray) and Sylvester (Scott "Carrot Top" Thompson) who try to talk him into buying a "rare" root used to make tea to make people younger. George sees that Sylvester a seemingly old person can perform marvelous feats of youth and agility but does not know that Sylvester is a young man only impersonating to be an old person. Mr. Wilson is about to pay $10,000 when Dennis comes by and turns out to own a root of the same kind, which he says he found on a place where those abound.
Soon afterwards, the two impostors return and sell Mr. Wilson a machine (for which Mr. Wilson paid a $15,000 deposit) that allegedly makes people younger. Suddenly, the attitudes of him and Mr. Johnson reverse as the latter feels George's pain of living in the same neighborhood as Dennis, while he starts to feel youthful and happy.
While Dennis is trying to clean up a pile of garbage that he accidentally threw on Grandpa's car while he was taking out the trash, he accidentally destroys Mr. Wilson's machine. As a result of this, the Wilsons plan on moving away to be away from him for good, whereupon Mr. Johnson decides to move into their house, although no one seems to really want to carry out this plan.
Overhearing everything, the Professor and Sylvester decide to use George's plan as an opportunity to get more money from him. They pretend to be several different workmen at the Wilsons' house when they were planning to move. The con men attempt to yet again to drain his bank account by stockpiling a hoard of his as yet not cashed checks by claiming that the house needed several repairs (termites, pipes, heating, foundation and so on) before it could be sold. Dennis helps the police (unintentionally) catch them. Mr. Wilson is hanging off the ledge of the attic when scared by one of Dennis's critters. Dennis goes to help and scares off the con men, who consider him bad news. In the process, the con men end up falling through their own construction site. The police tell Mr. George that the repairmen are in-fact con men.
The police return the not cashed checks, and George decides not to move after Dennis begs him not to do so. Mr. Johnson, however, announces intentions to get his camper back, having promised to take Dennis to the Grand Canyon, also because of everything he has put him through. Dennis, who was a "menace" throughout the whole movie, ends up being a hero.
Dennis and Mr. Johnson are in his camper in the Grand Canyon and Dennis, wanting to take a rock home to George as a present, accidentally takes the one from under the camper, causing it to roll down the incline it is parked on top of with Mr. Johnson still inside. While enjoying the Christmas season, George and Martha find out what has happened on the news as Dennis explains to the camera and Mr. Johnson is being airlifted to safety. Dennis gives a shout-out to George, which leaves him so flabbergasted that he mutters to the viewers, "He's a menace!".
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