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. 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 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.
Snowman Madness: The Mitchell family is on vacation at a ski resort where an ordinary day of Dennis and his father building a snowman leads to various misadventures. The Invisible Kid: Joey messes with PeeBee's latest formula, causing him to become invisible, except for his feet. Home Destruction: Henry decides to build a new playroom for Dennis as an extension to the house.
Up, Up, and Away (from Here!): Henry and George enter a hot air balloon race, but Dennis and Joey decide to join Mr. Wilson's team, leading to high-flying hilarity as George gets stuck on Henry's balloon as the boys take Mr. Wilson's to victory. Going Ape: Dennis, Joey, and Margaret all get a ride with PeeBee on his makeshift hot air balloon and then proceed to get lost when they get caught in a storm. They wind up on an island that's inhabited by a giant gorilla who takes a liking to Margaret. Dennis the Pirate!: The Mitchell family spends the day at the Pirate Land amusement park.
Hopping Mad: Dennis is invited to a music recital by Margaret and his parents give him money to buy a gift for her along the way. He stops to see a magic show, and the magician's assistant sells Dennis his boss' magic hat. Mayan Mayhem: Dennis and Mr. Wilson get onto the wrong plane from their families' intended vacation, ending up in the Yucatán, in Mexico, when they're forced to bail out due to excess weight. The Big Power Trip: Dennis and his father are on a tour of an electric plant when Ruff is spooked by the turbines and runs off.
Strong Medicine: PeeBee's latest invention causes Dennis to have super strength. Gold Strike: PeeBee and the Mitchell family take a trip to the Goldfield military base to view the test of a rocket plane Henry's company has built. Lights, Camera, Mud: Dennis, Gina, and Tommy are making a movie, and have Mr. Wilson play the lead as a mud monster.
Invasion of the Blob: An alien blob arriving on Earth in a UFO is scouting the planet so he can notify the following fleet if the planet is worthy of conquest. Wild West Showdown: While visiting a Wild West show, the Mitchell family catch a stagecoach robbery show, but Dennis and Ruff pursue the crooks believing it's a real robbery. The Hen Party: Alice is hosting a tea party for her friends and Mr. Wilson says he's not going considering the event is only for women, referring to it as a "hen party".
Up, Up, and Oh Boy!: Henry takes Dennis, Jay, and Margaret out to the field to test his company's new derigiblimp. The Company Picnic: Henry takes Dennis to his company's father/son picnic, and the two compete in the various games with the other fathers and sons, including a pair of troublemaking bullies. Aw Nuts!: Dennis wants to catch a squirrel to keep as a pet, but the crafty animal is able to outsmart him and Ruff at every turn.
Clip-Joint Capers: Dennis' barber rents a guard dog in an effort to prevent his shop from being robbed. Tanks for the Memory!: The Mitchells and the Wilsons are on vacation in Arizona, and get a tour of a military base. Second Honeymoon: The Mitchells take Dennis to the Silver Lake Lodge, where they spent their honeymoon.
A Couple of Coo-Coos: Mr. Wilson brings home a few carrier pigeons for the purpose of racing them. He and Dennis take one on an outing to see if she can find the way back to his house and the mission becomes for real after Dennis locks them out of George's car with the keys still inside. The Cloneheads: Dennis gets into PeeBee's latest invention, a clone booth, and it makes four duplicates of him. Nothing but the Tooth: Joey needs to have a tooth pulled out, but is afraid of the dentist.
Mummy's Little Boy: While Dennis and Margaret are playing doctor and nurse, with Joey as their patient, he gets wrapped up in too much bandage and causes the other kids to panic when they think he's a mummy. Horsing Around: While Dennis and Tommy are skateboarding in the countryside, they encounter a horse who takes a liking to Dennis and follows him home. Dennis Plasters Pamplona: The Mitchell family visits Pamplona, Spain, where Dennis learns about the running of the bulls.
Dennis Predicts: After he gets hit on the head during a game of kickball, Dennis has visions of events that haven't occurred yet, all of which become true. Dennis and the Kangaroo Cavalry: The Mitchells and the Wilsons visit Australia to attend the Royal Order of the Kangaroo Convention, in which Henry and George are members. Meatball Mess: After Ruff eats the leftover food that the family was to have for lunch, Dennis, Gina, and Joey do what they can with the remaining food (and other items) to make the special spaghetti meal Henry and Mr. Wilson wanted.
My Fair Dennis: Dennis goes to Upper Crustville to visit his rich aunt Olivia and cousin Chauncey and causes disasters with everything he does while at their mansion. A Good Knight's Work: When he falls asleep while watching a medieval show on TV, Dennis dreams of being a knight in the period era. Life in the Fast Lane: PeeBee invents a remote control time tuner that works on people instead of the TV.
The kids' favorite kid star, Nicky, comes to their neighborhood to make a movie.
Dennis is riding his bicycle with Ruff in the basket and accidentally knocks down Mr. Wilson. Poor ol' Mr. Wilson acquires a bad case of amnesia and can’t remember a thing, not even who he is.
Dennis' father Henry gets promoted to a new job in his business, only his job is in Alaska, and Dennis and his family have to move in one week. Features the episodes The Boss Gets Scalped, The Big Candied Apple, Going to the Dogs, Vampire Scare, A Royal Pain, Trembly Assembly, and Shock Therapy.