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. 

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.

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.