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.

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.

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.

They're shooting a "Jaws"-type movie at the beach, when the remote control falls into the mechanical shark's mouth, and it zooms off. Dennis finds the fake shark, and crawling inside it, has fun frightening his friends.

Dennis and his family visit Mt. Rushmore, which is in the middle of being renovated. Dennis scales the scaffolding in an effort to help out. There he meets an old man holed up in one of the president's nostrils.

Dennis goes fishing with Mr. Wilson, who's out to finally catch "Old Gramps", smartest trout in the whole lake. After a number of zany mishaps due to Dennis' over-eagerness, Wilson manages to hook the prize-winning fish -- only to have Dennis cause it to escape back into the water. When Mr. Wilson gets his hands on a radio that PeeBee has invented -- one that seems to... Read all

Mrs. Wilson has bought George a gift membership at a Health Spa. Dennis accompanies Wilson on his first visit, where he proceeds to drive poor Wilson nuts, speeding up his exercycle, pouring bubble bath in the spa water, etc. Finally the management tears up Wilson's membership.

Dennis brings home a stray kitten which Alice makes him turn over to the Wilsons. But the "kitten" is actually a baby mountain lion, which proceeds to wreck the Wilson's home. Things get worse when it's angry mother shows up, looking for its cub!. Mr. Wilson takes Dennis and Joey to the circus and ends up being the main event.

Dennis is made the prop man for the community theater whereupon he turns a serious drama into a roaring comedy. When Henry takes Dennis and the gang on a tour of his aerospace company, the gang slips away. They find a rocket ship and decide to investigate. Once inside, Dennis hits a button and WHOOSH! they are blasted into outer space.

When Henry and Alice leave to go to a "Poor People's" charity auction, Dennis decides to help the poor people by selling most everything in the Mitchell household.

Slippery ice causes chaos when Henry and Dennis set out to make a snowman. After tumbling and sliding all over the ski resort, they end up as part of a snowman and receive hot cocoa for their efforts.

Dennis keeps his eye on the secret solar powered moonwalker for his Dad. But when Joey touches the wrong button, it takes them for a stop-and-go-ride around town, eventually leading two crooks who want to steal it to the police. Dennis and Joey build a raft to go down the local river.

Dennis visits a supermarket and completely dismantles it. The Mitchell's visit New York. The Statue of Liberty will never be the same. The gang goes to the beach and decide to look for treasure with a metal detector. Once there, a bully tries to take it away from them. Ultimately, Dennis saves the bully from danger by way of the metal detector's powerful magnet.

Henry's father comes to visit, and he tells Dennis what Henry was like as a little boy. Not surprisingly, the child-sized Henry and the present-day Dennis had a lot in common. They both create chaos wherever they go. Dennis and PeeBee enter a friendly challenge of building and flying their own airplanes.

Dennis, George, Henry, and Joey all enter a balloon race. Dennis wins the race, but only after a wild series of adventures. A take-off on King Kong, in which PeeBee, experimenting with a gas-filled balloon, loses control of it, and he, Dennis, Margaret, and Joey, are carried off, ending up on a banana-shaped island.

When Ruff upsets one of PeeBee's experiments, the dog and Joey end up with their brains switched, Ruff can speak, while Joey can only bark! PeeBee and Dennis chase them around town, trying to capture them and re-switch their brains back to their owners.

Mrs. Wilson will not return until Mr. Wilson gets rid of all the mice. PeeBee's special mousetraps are regarded as circus rides by the mice and they come from everywhere to play, which costs Mr. Wilson a lot of quarters, the price he pays to the boys for each mouse captured.

Dennis and his parents visit the beautiful ranch of his favorite Uncle Bud. Uncle Bud has always wanted to win the local rodeo, and Dennis helps him out by riding the wildest bronc in six counties!. Henry's company has developed a sophisticated self-inflating tent to be used by space explorers on the moon.

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Dennis buys a magician's hat and gives it to Margaret at her cello recital. The hat goes wild, spewing frogs and rabbits all over the place, creating all kinds of comedy. Dennis and George Wilson take the wrong vacation plane and end up in South America, being pursued by Mayan natives. They have all kinds of wild adventures trying to get back home.

Dennis is blasted by a "strength ray" that Peebee has invented, and suddenly has super muscles. he goes around using his new-found strength to do various good deeds; rescuing kittens from tall trees, etc. Dennis challenges the town bully to a fight, not realizing that his super strength has worn off. But the bully proves to be a coward, and runs off anyway.

A blob from outer space enters various forms tree, Ruff, Alice, Henry, Mr. Wilson, baseball in search of a good place to plan an invasion. But Dennis manages to make the blob uncomfortable no matter where it hides.

On a bicycle-built-for-two Henry and Mr. Wilson race after Dennis, Margaret and Joey who have accidentally taken flight in Henry's newest invention, a dirigi-blimp.

After making the barber's guard dog sick, Dennis volunteers Ruff for watch dog. When bank robbers next door drill a hole in the wall of the barber ship to escape through, Dennis and Ruff capture them but make a shambles of the barber shop.

Dennis accidentally locks the keys in the car while out in the country with Mr. Wilson for a demonstration of homing pigeons. Dennis ties a note to the pigeon which gets stuck in a tree and then Mr. Wilson gets stuck after freeing the bird.

Wrapped head to toe in bandages while playing hospital, Joey gets picked up by an ambulance and taken to the hospital. Dennis and Margaret attempt to rescue him and make a shambles of the hospital corridors. Dennis makes friends with an old horse named Horace.

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After Dennis gets his father stopped by a policeman, Ruff swipes the cop's hat and Dennis and the cop chase him through the city streets leaving a mess in their wake. Dennis stops a bank robber when Ruff is found and is made an honorary policeman.

Mischievous Dennis Mitchell (Mason Gamble) makes the life of neighbor George Wilson (Walter Matthau) miserable with his overactive energy and inadvertent troublemaking.

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