Dennis' grandfather is laid up sick, so Mrs. Mitchell leaves to take care of him for a week. That means Dennis has to stay with the Wilsons while his father is at work. This is pretty much Mr. Wilson's worst nightmare, and he's right; Dennis gets into everything. In the evenings, Dennis and his dad hold down the fort, but it's tough going.
The weather is scorching hot, but Dennis has an idea! He convinces his family to put up Christmas decorations and pretend it's cold outside. Mr. Wilson witnesses all this and realizes that, yup, the neighbors are crazy.