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. He rides him in the Town Derby, and they win spectacularly. The Mitchell's visit Pamplona, Spain just before the running of the bulls. But Dennis accidentally releases the bulls a day earlier, and Henry finds himself chased into a bull ring. He fights off literally hundreds of bulls, becoming the greatest matador of all time.

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. Mrs. Wilson arrives with the fire department after seeing him on the news. PeeBee's newest invention accidentally makes four more of Dennis to the terror of Margaret, Mr. Wilson, and his parents. PeeBee and the original Dennis lure the clones back into the invention with an ice cream truck bell and the neighborhood is restored to "peace.". PeeBee invents a robot tooth-puller to extract Joey's tooth. They practice on Mr. Wilson, removing his dentures, which terrifies Joey straight to the dentist's office. Realizing where he is, he heads for the door where Dennis accidentally knocks him down and the tooth falls out.

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, his parents, and the Wilsons visit an Army base in Arizona. Somehow, Dennis and George Wilson become trapped in a runaway, computerized tank...which is headed straight for the Grand Canyon!. The Mitchells return to the lodge where they spent their honeymoon. Dennis brings his frog and causes so much disturbance that they are asked to leave. Henry and Alice don't mind, preferring to keep their memories unclouded.

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. The invasion is called off since the blob feels a planet full of people like Dennis is unconquerable. Henry and Alice take Dennis to a western-type theme park, where he sees a re-enactment of a stage-coach hold up. Think the hold up is real, Dennis leaps aboard the getaway stagecoach to stop them, and ends up at a boarded-up Ghost Town Saloon. There Dennis, using the available props in the "haunted" saloon, frightens the cowboy actors into turning themselves in to the authorities. It turns out they really were hold up men, using the wild west show to cover their tracks. George tells Dennis that Martha and Alice are going to have a "hen party" with a few of their friends. Trying to help out, Dennis brings a few chickens to the affair, which causes instant, mass confusion.

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. The Mitchells and PeeBee visit Gold Field Air Force base in the desert to watch a test flight of a rocket plane developed by Henry's company. More interested in finding gold, Dennis and PeeBee stumble upon some spies who are planning to explode the plane. The boys' quick thinking brings help and foils the spies' plans. Dennis is directing a movie about a "Mud Monster" that attacks the neighborhood kids He asks Mr. Wilson if he'll play the creature. Wilson's vanity gets the best of him and he agrees. Unfortunately for him, the scenes call for him to take a terrible beating... not to mention being covered in mud. In the end, Wilson, trying to get home, accidentally drives his car in the swamp they were shooting around.

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, Ruff, and Henry visit a city power plant. Ruff gets loose on the big control board, and Dennis chases him. They hit every power switch on the board, turning the entire town into a pinball machine.

After Dennis reads a medieval fantasy book, Dennis and Joey are sucked into a fantasy land similar to the book he read. They are charged by the Elvin King with defeating a wicked monster; The Boys go camping for the weekend in his father's boss's mother home. Dennis wishes to keep one as a pet; Mrs. Wilson leaves to spend the day at her nephew's. Leaving Mr. Wilson home alone. Dennis terrorizes Mr. Wilson with his remote controlled airplane. George gets hit on the head, he then thinks Dennis is his favorite nephew. He then takes Dennis and Joey out for a day of fun.

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. An Army General, anxious to see if the tent actually is "simple enough for a child to operate", insists that he be allowed to borrow Dennis for an over-night camping trip. Before the camping trip is over, Dennis has the poor General cornered by a hungry grizzly, chased by a swarm of angry hornets, and attached by a love-sick moose. Returning the following day, the General requests less dangerous duty... like joining the Green Berets. Dennis' friend PeeBee invents a formula which actually grows hair! When Mr. Wilson find out about it, he chases them through town, trying to get his hands on the stuff, and becomes wealthy. But when he finally finds them, the bottle smashes, gone forever!

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. A take-off on the "Wizard Of Oz", in which Dennis and Ruff are carried off to Munchie-Land by a cyclone... there they meet Joey, PeeBee, and Gina, each looking for Courage, a Brain, and a Heart, respectively. Before reaching the City of Odd, they escape the clutches of Margaret, the wicked Witch. Finally they find the Wizzer, who looks a lot like Mr. Wilson, and Dennis wakes up, back in his own bedroom. Dennis and Joey go into the dog washing business and follow Mr. Wilson to the car wash. Halfway through the cycle Dennis opens the car door and a parade of wet animals soak Mr. Wilson's car followed by the jet spray of the rinse cycle.

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. The kids meet a looney Professor at the Science museum who has built a time machine that is ever so close to actually working. When their teacher tells the kids that they must have a science project turned in the next day, Dennis gets the wild idea of going back in time and taking actual photographs of actual dinosaurs. They're wild idea is made possible when PeeBee reworks the Professor's calculations and makes the time machine work! WHOOSH! They go back to the prehistoric age. While they befriend a baby dinosaur, the problem is that the time machine is sinking in quicksand and the mother dinosaur is chasing our heroes! Fortunately Dennis and the kids are able to retrieve the time machine and return home with the photos... unfortunately, they contain NO dinosaurs!. Dennis challenges Margaret to a soap box race, building a homemade car with Joey's help... and loses.

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. There they meet a giant ape, who falls instantly in love with Margaret. Returning home by raft, they put the giant ape in show biz, doing a ballet dance duo with Margaret. During the show, the ape panics... and so does the audience! The ape climbs a high tower, and falls. But the balloon the kids have sent up catches him, and he's floated safely back to his island home. Dennis and his parents visit Pirateland and take the Pirate's Run, the wildest ride in the park. Dennis takes charge of the ride and throws the entire amusement park into chaos.

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' turns out to be a rickety old thing, while PeeBee's makes a Lear jet look like scrap metal. Once in the air PeeBee flies rings around Dennis, making him look bad. However, in the end, PeeBee's plane malfunctions and Dennis has to rescue him. Dennis ultimately turns out to be both the hero and the winner. Dennis and Joey camp out in a treehouse in the Mitchell backyard. George decides to scare them, but he doesn't know the boys have rigged all kinds of booby-traps around the tree house, and he gets caught in every one of them.

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.

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. With Margaret as the unwilling guest, they proceed down the rapids, the Margaret forever watching out for her new hairdo. The kids are able to survive the wildest of adventures, and it isn't until they walk back home that Margaret finally has her hair soaked by way of Mr. Wilson's garden hose. Dennis and Joey go to a Cowboy Bob matinee to watch their favorite western hero right the wrongs of yesteryear. But they get into big trouble with a big bully. Dennis cries out for help...and Cowboy Bob steps off the movie screen to come to his rescue!

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. Joey gets soaked with PeeBee's newest solution and he disappears. His shoes escape the solution and the tow is terrified by a pair of shoes walking around. Mrs. Wilson suggests a bath might restore Joey to sight, which it does, but not before Mr. Wilson is duped. Dennis "helps" his father build a new playroom onto the house. Chaos reigns as Mr. Wilson laughingly looks on.

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.

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. Mr. Wilson, who is supposed to check on Dennis but arrives late, is shocked when he sees what Dennis is doing. So, he and Dennis go on a madcap scavenger hunt throughout the neighborhood to retrieve the pieces Dennis has sold before Henry and Alice get back home. By the time they get back with all of the pieces, Henry and Alice arrive with lots of "new" junk. So much so that Henry declares that maybe they should hold their own auction! Wilson falls down in exhaustion. Vacationing at a ski lodge, Dennis and Joey cause so much chaos that the manager dresses as a snow beast to scare them off. But when the beast gets into trouble Dennis rescues him and the guests vote for the boys to stay. Dennis and PeeBee, in an effort to help Dennis clean his room, decide to build a robot to do it for them. Once built, the robot escapes and causes havoc throughout the neighborhood. Mr. Wilson, who is being bothered to cut the lawn by Mrs. Wilson, thinks it's a creature from outer space and gets caught up in the chase. In the end, Wilson finds out that it's Dennis' creation and gets the bright idea of using the robot to cut his lawn!

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. There they encounter meteor showers, hyper-space, etc. Fortunately, the kids discover that they aren't in a real rocket -- but a "Flight Simulator!". Dennis finds an East Indian flute, that causes objects to rise and dance, snake-like. He walks through the town, playing it, and causing all sorts of weird things to happen to telephone wires, dishes of spaghetti, Wilson's flower garden, and the like.

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. His unwitting participation is a big success which spoils a former ringmaster's efforts of revenge. Dennis and Tommy set traps for the Mudville Swamp Monster while Margaret whines, but allows them to use her "tracking equipment." The monster scares Margaret into the trap, and when the Ranger arrives to help, he congratulates them for finding the missing hippo from the zoo.

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. Grateful that he won't have to go back, he buys Dennis, and himself, a huge fudge sundae. Henry is sent to Alaska to track down a crash-landed satellite. Dennis, Ruff, and Joey stowaway along on the trip, and after misadventures involving Eskimo kayaks, icebergs, etc., Dennis discovers the "killer whale" that supposedly swallowed the satellite, is actually a submarine, carrying 2 foreign agents, anxious to capture the top-secret satellite. Dennis, of course, puts a stop to their plan, and the baddies turn themselves over to the Eskimo authorities. Henry is forced to take Dennis along to the golf course where he is to play with his boss. Henry is trying his best to "lose to his boss," but Dennis doesn't understand this and tries his best to get dad to win. Golf madness ensues with Dennis going wild with the golf cart, other players, hazards, etc.

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 predict future event-his mind races with thoughts of great wealth. But a garbage truck has accidentally carried off the radio, and now Ä with Dennis along - Wilson races to follow the garbage truck. It's course leads to the beach, where Wilson gets involved with huge waves, skate-boarders and roller skaters...then finally to the city dump, where he goes through more indignities. Wilson finally recovers the precious invention-to find that it's an ordinary police band radio, without any special powers. Dennis knew about it Alice along, but didn't tell Wilson, because he was "having so much fun!" A stranger offers Dennis and Joey a dollar to deliver a package. But when the package starts ticking, the kids are convinced it's a time bomb, and try to get rid of it-but it keeps coming back! The "bomb" turns out to be a clock.

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 wants to help the old man. A chase ensues with the workers trying to catch Dennis. With the old man's help the scaffolding is blown away with dynamite and a fifth face appears on the mount... Dennis goes to the local lake with his parents to do some fishing. He is very happy to get away from Margaret for a few days-however, once at the lake he finds that she is also there on vacation with her parents! She coerces Dennis to row on the lake with her. Once out there, though, they are attacked by a "REAL" lake monster. They row to an island, and while there they encounter pirates, cavemen, erupting volcanos, et al. Fortunately, they discover that they are actually in "Crazyland" - a new amusement park!Dennis hypnotizes Mr. Wilson into being a 9-year old kid again, in order to play with him - but the kid Wilson turns into, is an insufferable brat!

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. The shark talks to amazed Mr. Wilson, who summons the press, claiming he's found a talking shark! Naturally, he looks foolish when Dennis climbs out of it. Henry takes Dennis and Joey to a drive-through Lion Country Safari-type place... then, thanks to Dennis, the car breaks down. He locks the kids in, while he heads back on foot, making them promise not to leave the car! But almost instantly little Joey is running through the jungle, totally unaware of the dangers around him, and soon Dennis has to rescue him from a charging rhino, a hungry crocodile, and an angry elephant, among others. But Dennis manages to get Joey safely back inside the car, only moments before Henry arrives with the gas -- never once suspecting that the kids had ever left it!. Dennis and Henry go to the Steeplechase race to watch a horse that Henry's friend owns. However, just before the race, Dennis, while trying to help out the jockey, adds a little too much in the jockey's "protein drink," causing the jockey to drop out of the race. Dennis feels responsible, so he takes off on the horse and enters the race. It is a wild race with Dennis coming out the winner, making the horse's owner very happy.

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.

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.