When Andy Anderson returns to school in November, he has a peg leg—and an erector set his dad bought him in order to get other kids to play with him now that he can't run around.
Tom's mental wheels start turning about how much money he could make if he owned an erector set.
Brownie and Lady's puppies are ready to leave their mother, so Tom takes J.D. to choose a puppy for himself.
Now we find out why Tom would lose a fortune if Lady were lost: Turns out he has a deal to sell the pups to Mr. Monaire, a local sheepman, as quality sheepdogs.
Tom gets two silver dollars for the male pup he was promised from the litter, and Frank and Allan Jensen each get a silver dollar for the remaining female pups. Wage inequality even extends to sheepdogs, we guess.
Even though J.D. is happy with his new puppy, he feels like Tom cheated him somehow, but he can't figure out how. Seems like Tom just didn't tell J.D. the whole story about why he wanted to mate the dogs.
But back to our story about Andy Anderson. After a couple of incidents where all the boys run off to play without him, J.D. finds Andy crying behind the barn.
Andy has decided to kill himself because he can't do any of the things he used to do, like play with the other boys or do his chores. He overheard his father say he is useless—ouch.
J.D. agrees to help Andy kill himself (like a true friend), but the two of them together can't get the job done. They try drowning, consider bleeding, and are just about to take a crack at hanging when Tom happens upon them.
Shocked that Andy really wants to kill himself, Tom offers to put his great brain to work to figure out how to keep Andy from being useless—in exchange for his erector set.
Tom starts by teaching Andy Indian "squaw" wrestling. Yeah, not the most PC name for a game, but again, 1896. He uses the old trick of making Andy believe he's stronger than he is, which leads to Andy's eventually becoming stronger.
Then they practice Duck on a Rock, a throwing game.
Andy asks about his chores—now that he can't do them, he really wants to be able to—so Tom decides they'll spend half of each day on chores and half on games. Of course, Andy needs some practice doing chores, so Tom lets him do his and J.D.'s. for a whole week. Such a team player.
It all works out for Andy because his father says he is proud of him.
Feeling that Andy won't be any good as a kid until he can run, Tom decides to teach Andy to run.
It takes him a week, but he does it, and then he teaches Andy to bat with a peg leg.
Finally, Tom is ready to show off all his hard work. He gathers all the boys. Andy beats Sammy at wrestling and wins Duck on a Rock; then he wins a race and plays in a baseball game in which he hits a home run.
That evening, Andy arrives at the Fitzgerald home with the erector set, explaining that he told his parents about the deal and everything Tom did, and they said it was all right.
Wait for it, because here's the big twist: Tom doesn't take the erector set. The Great Brain actually refuses payment—he says it doesn't feel right.
J.D. is ready to call the doctor, but Tom says he feels fine.
Tom even takes off the real Indian beaded belt and returns it to J.D., who still thinks he must be sick.
No, Tom is just overcome with the foreign sensation of feeling good because of doing good.
He reforms, but J.D. feels that makes things pretty dull.