The narrator, seven-going-on-eight-year-old John D. Fitzgerald, introduces the setting (Adenville, Utah, 1896) and his family.
There's Sweyn D., who's going on twelve, and Tom D., ten, who is also known as the Great Brain.
As some of the only non-Mormon kids in town, the Fitzgerald brothers have gotten really good at beating other kids up in the name of tolerance and understanding.
Adults are present in the form of Mamma and Papa, who is the editor of the Adenville Weekly Advocate. There's also Aunt Bertha (not their real aunt) who lives with them and makes wisecracks.
For an otherwise smart man, Papa has one weakness: He can't resist buying amazing new inventions, very few of which actually work. He's like an old-timey Muggle version of Arthur Weasley.
Papa's newest acquisition is an indoor toilet, otherwise known as a water closet.
The whole family is pretty sure this is going to be a disaster, but the Great Brain sees a way to make a buck—or forty cents, anyway.
Tom D. starts charging kids a penny to sit on the porch and watch Mr. Harvey, the plumber, dig the hole for the cesspool in the backyard. Yep, this was entertainment in 1896.
By the way, the boys all have the same middle name, Dennis, and call each other by their initials, as does their dad. Turns out, the original Dennis was a traitor to his family during a rebellion in Ireland a century before the time of the story. So now all the boys are named Dennis to remind them of that time that one Dennis really sold everybody out. Makes sense, we guess, if you want the opposite of being proud of your ancestors.
Tom keeps this scheme going for the two days it takes Mr. Harvey to finish the cesspool. He even goes to Zion's Cooperative Mercantile Institution, otherwise known as the Z.C.M.I. store, to buy cookies to include in the price of admission. Dude is an entrepreneur.
When the water closet arrives, Nels Larson, who works at the train station, makes sure everyone in town knows.
The whole town turns out to see the water closet unpacked, but Mr. Harvey draws the line at watching him install it—a plumber must have some secrets, after all. Papa gets everyone out of the house by telling them they can see a demonstration after it's in.
So everyone sits around on the lawn while Mr. Harvey works. It's a real event.
John D. is so embarrassed about the water closet that he runs upstairs to cry on his bed until he hears a gigantic noise.
Turns out water closets were noisy back in the day—no secret flushes here.
The adults in town all get a chance to file by and watch the water closet flush, but it's too late for the kids; there's none of this children first business in 1896.
Tom sees an opportunity in this exclusion and brings J.D. in as a business partner. J.D.'s job is to stand on the street with a sandwich board and bring in business by ringing a cowbell.
Tom charges the kids to see the water closet until Mamma gets wind of his scheme and makes Tom and J.D. return all the money. Tom is devastated by this financial setback.