It's 1896 in Adenville, Utah, a town of two thousand Mormons and five hundred Protestants. It's the year Utah becomes a state (not important to this story), the year the Fitzgerald family gets an indoor toilet, and the year Tom D., the middle Fitzgerald brother, realizes that using his Great Brain to help others makes him feel good even if he doesn't make a penny off it.
First, though, he makes a lot of pennies.
Narrated by eight-year-old John D., often called just J.D., the youngest Fitzgerald, The Great Brain is a series of connected episodes about Tom D.'s self-interested schemes. Starting off with a story about the time Tom (a.k.a. "the Great Brain") charges his friends admission to watch a plumber dig a hole in the backyard for a new flush toilet, the Great Brain's exploits only get bigger and more daring:
See what we mean? The Great Brain is a schemer, for sure, working other people's misfortunes to his own advantage and earning a pretty penny in the process.
In the middle of the Great Brain's shenanigans, a poor Jewish storeowner starves to death without anyone realizing what's going on, prompting the townsfolk to do a little self-reflection on their own selfishness and religious discrimination issues. Unfortunately, they don't seem to do anything more than talk about it.
In the second half of the book, eldest brother Sweyn D. heads off to a Catholic boarding school in Salt Lake City, leaving the Great Brain and John D. behind.
The beginning of the school year brings a few new surprises. The boys' friend Andy Anderson doesn't come to school because he stepped on a rusty nail and now has to have his leg amputated. (Yes, that's what happened when you stepped on a rusty nail in 1896.) Also, there's a new teacher in town, Mr. Standish, who dares to paddle the Great Brain—with an actual paddle. (That's what happened when you acted up in school in 1896.)
The Great Brain swears revenge and orchestrates a plot to convince the town's grown-ups that Mr. Standish is a secret drunk. This is Tom's greatest scheme yet, and it almost works until Papa figures it out and Tom has to confess. He still comes away from the experience smelling like a rose, though, even earning Mr. Standish's eternal gratitude and the respect of his peers all in one go.
In November, Andy returns to school, very glum because he can't play a lot of games or do his chores with his new peg leg. Andy does, however, have an erector set—an erector set which catches the eye of the Great Brain.
You know where this is going, right? Tom agrees to teach Andy to play games and do his chores on one leg in exchange for the erector set. Tom succeeds beyond anyone's wildest dreams, and then, when Andy thanks Tom for helping him get his life back, the unthinkable happens: Tom realizes that he actually likes doing good. In fact, helping others makes him feel good. Whoa. Then, because it's Christmas, he lets Andy keep the erector set.
Anachronism alert: Erector sets didn't come along until 1913, but we'll let Fitzgerald get away with it (source).