This film is about a boy named Jay Berry Lee. Jay finds out from his grandfather that a circus in the area is offering a reward for the return of 29 monkeys that have escaped from a wrecked circus train. Jay's father and grandfather help Jay and his blue-tick hound Rowdy attempt to catch the monkeys, intending to buy a Paint-Pony and a .22 rifle with the reward. However, one monkey, Jimbo, is a genius and thwarts Jay Berry's plans.
Summer of the Monkeys takes place in the very late 19th century. It is about a fourteen-year old boy named Jay Berry Lee who wants a pony and a .22 caliber gun.
The book starts by describing Jay Berry's happy and idyllic life, which is ruined when he discovers a batch of monkeys. Jay Berry's parents were young and proud sharecroppers living in Missouri, but they were also somewhat unhappy because people looked down on them. Then they had twins: Jay Berry and Daisy. One day, Jay Berry's mom received a letter from her father (Grandpa in the story), saying he gained 60 acres of land by trading and wants to give the land to them. They accept the offer and move to the Cherokee land in Oklahoma.
The book tells about Daisy's crippled leg, and how hard the family is working to get her an operation, and how much Jay wants a pony. Jay Berry has a long-time companion, a dog, called Rowdy. The family milk cow, Sally Gooden, gets lost. Papa tells Jay Berry to go look for her. As always, he finds her in the river bottoms, but he also finds a monkey while on the way there. He tells his father, who somehow believes him. Papa tells him that Mama wants him. Mama tells him to go to his Grandpa's store for groceries after Jay Berry tells her about the monkeys. Grandpa tells Jay about a reward that is being offered for catching the monkeys alive. $100 is offered for a brilliant monkey named Jimbo and $2 for each little monkey, which was a great sum of money in the late 19th century. Since there are 28 little monkeys and one big monkey, the total is $156. Jay decides he will try to catch the monkeys. Grandpa gives him traps he made which were designed not to hurt the monkeys.
In his first attempt to catch the monkeys, he sets the traps under a tree, hides them, and pins apples on the tree. This plan fails when Jimbo simply reaches high and gets the apples off the tree, and, when he is satisfied, he gives the rest of the apples to the little monkeys. For his second plan that day, Jay sets the traps, hides them, and places apples on the trigger. This plan also fails when Jimbo uses a stick to trip the traps. He again shares the apples with the little monkeys. Later, Jay Berry decides to take a break and heads to the nearby spring. When he gets back, he finds that the monkeys have stolen his lunch and traps. He gets angry and shoots rocks at the monkeys. The monkeys then get very angry and chase Jay home.
The next day, Jay Berry goes to consult with Grandpa. Grandpa gives him a butterfly net that can open and close using two pull strings and tells him to dig a hole that night so he can hide from the monkeys to try to catch them. He catches two monkeys, but Jimbo releases them and orders the small monkeys to attack Jay and Rowdy. They come out of the fight with many scratches and bites. His sister wants to be a Red Cross nurse when she grows up so she gives Jay her "Red Cross treatment" as she always does whenever Jay and Rowdy are injured.
After recovering, Jay Berry goes to talk to Grandpa again. Grandpa tells him that he wrote to the circus asking them for advice on how to catch the monkeys. The animal trainer responds by telling them to call Jimbo by his name and try to make friends with him. With this in mind, Jay goes back to the river bottoms. However, when he arrives, he is astonished to find the monkeys drunk on whiskey at a still. Jimbo offers Jay the mixture since he no longer sees him as a threat, and Jay reluctantly accepts. Jay wakes up in the middle of the night feeling drunk. He unsteadily goes home, where Daisy notices he is drunk and tells Mama and Papa. They tell Jay never to get drunk again. Once again Jay receives the "Red Cross treatment".
Grandpa decides to take Jay Berry to the town of Tahlequah to go to its library and do research on how to catch monkeys. They find out to use coconuts and a trap cage, and they go to the general store for these. As they return, they stop by a spring. When they return to the buggy, they find that the monkeys have stolen the items they bought.
Later on Daisy finds a fairy ring which is capable of granting wishes. The book has her wishing something, but it doesn't say for what. It turns out that she wished for Jay Berry to get his pony and his gun. Then, there is a storm. The monkeys are scared enough that they go with Jay to his house. Jay Berry gives the monkeys to the circus. The owner gives him lifetime passes to their circus and the $156 reward. This is enough to buy his pony and .22 or to get Daisy's leg operation. Jay almost buys the pony of his liking, but decides to use the money for Daisy's leg operation. During the many weeks Daisy and Mama are gone for the operation in Oklahoma City, Jay Berry and Papa miss them and struggle to handle daily chores without them. While Daisy is in Oklahoma City, she buys Jay a .22 which she gives to him during her return. Also, Grandpa buys Jay the pony that he was going to buy. Daisy and Mama come home on the train. Once they get off the train, Daisy kisses Jay Berry on the lips because she is so happy. When they all get home, She and Jay Berry run around the hills together, holding hands.