Dennis, Tommy and Stewart start a club house in Mr. Wilson's crawl space. After a fortune teller tells Mr. Wilson that he will hear from one of his late relatives, Mr. Wilson begins to believe that one of his late relatives has come back from the dead when he hears noises in the house that are caused by Dennis and his friends in his basement.
Dennis has a tree house that hangs over Mr. Wilson's back yard. Mr. Wilson is not thrilled about it cause he thinks with Dennis up in the tree house will scare all the birds away so he tries to teach Dennis about bird watching which in the end turns out good when Dennis takes care of a sick bird and helps Mr. Wilson get an award from the state bird society.
Mr. Wilson pays $250.00 for a rare gold coin from a con artist that turns out to be a fake coin. The good news is that the coin gives Mr. Wilson a free wish when Dennis throws the coin in the fountain in the park, Mr. Wilson wishes that Dennis would go on vacation for a week and his wish comes true in the end when he learns that Dennis is going to his Grandpa's for a week.
Dennis wants a bike so bad that he will do almost anything to get one such as taking one away from another child, Trying to win one in a bean contest, riding Mr. Wilson's exercise bike. Finally in the end Henry takes Dennis shopping for a bike.
The Mitchell's are having a open house party to entertain one of Henry's clients but people are having trouble finding the house, that is until Dennis finds an open house sign and puts in in the yard and soon the house is over run with people who think that the open house sign indicates the house is for sale.
Mr. Wilson is plagued by a full-grown duck that Dennis adopted when its original owner moved away. At first Dennis hides the duck from his parents, but when Mr. Wilson complains about the damage caused to his garden and himself, Dennis reluctantly admits to having the duck. His father helps Dennis build an enclosure for the duck, but Mr. Wilson continues to have problems with Dennis' new pet -- and begins to scheme how to get rid of it even if Dennis and his parents won't.
Dennis wants to put up a swing in his yard and ends up chasing a cat who Mr. Wilson is house sitting for a friend up a tree in the process. Meanwhile Henry tries to take Alice out for their anniversary which Henry completely forgot about. the ice cream man tries to get the cat out of the tree and ends up with his ice cream truck shut off so the ice cream man puts all of his ice cream in the Mitchell's ice box.
After Dennis meets Mr. Wilson's sister Helen and she reads the book "Treasure Island" to him and his friends Dennis decides he wants to be a pirate and dig for buried treasure, So, Mr. Wilson draws a map of his yard for Dennis and his friends to find so they can dig for where Mr. Wilson wants to plant his flowers. When Dennis and the other boys start digging they find a treasure only it came off a costume bracelet of Alice's that Henry put there for the boys to find. But, when Mr. Wilson sees it he starts getting excited until he finds out that it isn't real treasure.
When Dennis is over at one of his friends he notices that his friend has his own TV set so Dennis is doing anything he can to convince his parents to buy him a TV set of his own, However when Henry and Alice both turn him down he has Mr. Wilson show him how to work his remote control which was a mistake cause then Dennis rents a remote of his own and ends up controlling Mr. Wilson's TV set from the guest room with the remote that he rented.
Dennis wants his dad to be a hero so bad after Johnny Brady's dad always seems to get his picture in the paper for some heroic act. So Dennis tries to talk to the town reporter to put Henry's picture in the paper and make him a hero.
Dennis and a bunch of other town boys are collecting papers for a charity and the prize for the boy who collects the most papers is a Silver dollar that's worth $600. When Mr. Wilson gets word of that he is determined to make sure that Dennis wins the prize so that he can get the silver dollar.
To help the bees find honey Dennis sprays a fragrant smell on Mr. Wilson's camellias, Mr. Wilson of course gets carried away thinking he has developed a rare flower that will make him a fortune only to discover that the fragrant smell is from the Dennis spaying perfume on his flowers.
While Henry is out of town Dennis wants his mom to have a nice birthday with lots of presents. Mr. Wilson sends him down to Finch's drug store to shop for his mom. When he goes down to Finch's drug store he ends up causing chaos down there with Mr. Finch and to make matters worse he puts a sign on him advertising people to get his mom lots of birthday presents.
Mr. Wilson has a yard full of starlings that he desperately wants to get rid of so he hires a professional named Mr. Prince to do the job that has a record that will get rid of them. However, thanks to Dennis the record brakes so when Miss Cathcart starts singing the starlings start to fly away and to finish them off Dennis puts liver in Mr. Wilson's tree which eventually brings tons of cats.
After Dennis hears that the city is going to tear down Mr. Dorfman, the postman's bench Dennis, Tommy, Margaret and Stewart try to build him a new one but Dennis gets sick and has to stay in the house and watch the other 3 try to build the bench.
After Joey has to go home Dennis has no one to play with Henry is going golfing, Mr. Wilson is going bowling, Alice has a date to play bridge and his other friends aren't around So, Dennis decides he wants to go visit his Grandfather which is far away so when he asks Alice to take him to his Grandfather's and she can't Dennis decides to start walking there but, he doesn't get far as he ends up getting stopped by the police.
Dennis and Tommy try and sell Valentine's day cards in July to buy swim fins. However, it isn't long before chaos hits the neighborhood as it causes problems for Miss Cathcart, Mr. Dorfman, Mr. Wilson and even the gardener Mr. Carlson. There also ends up being a run on Valentine's day cards in July. Mr. Wilson buys the rest of Dennis' cards hoping to avoid any more trouble but it ends up with Mr. Wilson in trouble with the police as they think he was the one responsible for all the trouble with the Valentine's day cards.
Mr. Wilson's former employers offer him an executive position at his old company, but it will require him to relocate to Pittsburgh, PA. Dennis doesn't want him to go, but when Mr. Wilson decides to take the job, Dennis blames himself and runs away from home.
While watching an auction from the sidewalk, Dennis causes Mr Wilson to raise his hand when an old-fashioned radio is being auctioned. George has to pay for it since it appeared to be a bid. Turns out the radio has a hidden stack of money.
When George gives away books, he includes a book that holds a stock certificate worth $500. He tries to get the book back, but it is too late. Dennis saves the day when he realizes he found the stock and kept it because it looked neat!
With his father away on a business trip and his mother sick in bed with a cold, Dennis has to be the "man of the house". This includes doing laundry with Margaret's help, and arranging a dinner party with his friends in connection with a sales pitch.
The town holds a Paint-up, Clean-up Week campaign, and everyone spruces up their place. Dennis gets into the act with paint from Mr Wilson to paint his red wagon. Dennis paints the wagon and the street corner. Mr Wilson gets blamed.
Dennis is sure that he's going to get a horse for Christmas, even though his parents repeatedly tell him that he isn't (and Mr. Wilson would be sure to object). Then Dennis hears that another boy in the neighborhood has gotten a pony -- and he heads there to claim it, certain that Santa must have delivered it to the wrong house.