When Dennis overhears and repeats part of a conversation between John and Eloise in which they decide to no longer play cards together, rumors around the neighborhood begin to fly that the Wilsons are splitting up. With the belief that Mr. Wilson is now single, Miss Cathcart and Miss Tarbell compete for his affections with the help of Dennis.
With Dennis needing to earn a dollar to buy a toy laser gun but not being old enough for a job delivering circulars for Mrs. Elkins, Mr. Wilson pulls some strings to get Dennis the job (and also get him out of his hair). After quickly finishing the job, some damning evidence leads everyone to believe that Dennis took the money without doing the work.
Believing that Mr. Wilson is broke, Dennis goes all out to help his friend. This includes buying groceries for the Wilsons and leaving them on their doorstep; and putting on a show with his friends, using the admission fees to make money for Mr. Wilson.
Dennis, along with his parents and the Wilsons, visits the San Diego Zoo to bring back a chimpanzee for their town zoo. However, the chimp causes a ruckus at the motel where they are staying overnight.
While still in San Diego, the Wilsons receive a visit from their nephew, Ted, a young sailor. When Dennis and Mr. Wilson get stuck in back of a laundry truck returning to the Naval base, their attempt to escape leads to the confusion of Mr. Wilson's being mistaken for an expected demolitions expert.
John Wilson has taken over writing the advice column for the local paper. However, complications arise when he unwittingly gives advice to two women - Miss Cathcart and Miss Tarbell - who are out to catch the same man, Sergeant Mooney.
Henry does not want to play golf with his boss and hopes it will rain. He is even willing to pay for it. Dennis and Tommy perform rain dances to get it to rain despite the fact that it has been rainless for ten days.
Mr Wilson reads in the paper that a monster has been seen in town. Since his editor wants exciting stories, Mr Wilson decides to add to the story when he sees large footprints in the back yard. Dennis has a new novelty toy - large fake feet
Stretch Nichols, the school basketball team's star player, quits the team after being teased about his height and worn-out clothes one time too many. A sympathetic Dennis persuades Mr. Quigley to help by buying Stretch new clothes and giving him an after-school job, so that Stretch will feel better about himself and rejoin the team in time for the big game.
A neighbor's cat is bothering Mr Wilson; he thinks cats are a nuisance. When he finds out there is an ordinance that requires cats to be licensed, he unwillingly volunteers to do the job of rounding them up.
While his parents are out of town, Dennis has to stay overnight at the Wilsons' house. John Wilson is not pleased about this. And the added complication of a burglar in the neighborhood is making him even more nervous.
Mr Wilson has a chance to write a biography of a famous author by pretending he is a young and vigorous. When the actress visits, he needs to dye his hair black to look younger. Dennis & Tommy mix up the bottles of hair dye.
Henry gets a bonus at work, and when airline tickets to Hawaii arrive in the mail, Alice thinks the tickets are for Henry and her. However, Mr Wilson purchased the tickets as a surprise for Mrs Wilson and had them mailed to the Mitchells.
The first mockingbird of the season stays in a tree in Mr Wilson's backyard. Mr Wilson decides to run for president of the Birdwatchers Society, but has conflicts when he tries to run the bird off from his property.
Henry has to go on a business trip, and he tells Dennis he has to be the man of the house. Meanwhile, Mr Wilson tries to get a local store to display the book of articles he has written. All sales of the book would go to the Red Cross.
Concerned that Eloise will be destitute if he should pass away suddenly, John Wilson hopes that his Aunt Emma will leave her money to him, so that he can in turn provide for Eloise. However, when Aunt Emma and Dennis become friends, Mr. Wilson fears that Dennis will inherit Aunt Emma's money instead.