In a case of mistaken property, Dennis takes Mr Wilson's lawn edger to give his father. When Mr Wilson finds his edger is missing, he takes one from Mrs Elkins.
In order to claim a prize, Mr. Wilson must make good on his claim that he has four sons. So he recruits Dennis and three of Dennis's friends for the roles.
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.
Mr Wilson develops an allergy and the doctor cannot figure out what is the irritant. Mr Wilson soon realizes it might be an allergy to Dennis since that is the only time he sneezes.
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 wants to interview several gypsies for an article he's writing, but the gypsies are suspicious of outsiders. So he disguises himself as a gypsy in order to meet with them.
Dennis is curious about what "falling in love" means. And the fights and reconciliations of three couples - his parents, the Wilsons, and a teenage couple - only makes him more confused.
Mr. Finch is having a contest in which his store's fifty-thousandth customer of the year will win five minutes of free shopping. Mr. Wilson is determined to be the winner.
When Dennis sees Mrs. Wilson knitting booties (for Mr. Wilson's golf clubs), he jumps to the conclusion that the Wilsons are about to have a baby, and innocently spreads the word all over town.
Mr Wilson goes back to grade school as a student with Dennis after he complains the city spends to much on frivolous items for school. He wants to see first hand where the city is wasting money.
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).
When Dennis is appointed by the mayor to go to Washington, DC to ask their state senator to make the local forest a national park, Mr. Wilson believes his own Washington contacts can help.
When Dennis becomes friendly with the Wilsons' house guest, a little Chinese girl, Margaret becomes jealous of Dennis's attention to the Wilsons' visitor.
Dennis and his classmates eagerly participate in a "Junior Astronaut" savings-stamp program. The child who buys the most stamps will win a trip to Cape Canaveral to meet an astronaut.