Chapter 1 introduces the reader to James, Aunt Sponge, and Aunt Spiker. At four years-old, James lived with his parents in a wonderful house along the sea, until his parents were eaten by a rhinoceros during a day trip to London. Soon after his parents' death, James was sent to live with his aunts, bringing nothing with him but a pair of pajamas and a toothbrush. Aunt Sponge and Aunt Spiker forced James to perform back-breaking labor, locked him up for punishment, and never let him leave the confines of their house and garden. From the top of the hill that the house was perched on, James could see woods, fields, and sometimes his former house. There was little for James to do in the house or garden, and the only thing that remained on Aunt Sponge and Aunt Spiker's unpleasant property was an ancient peach tree that never grew peaches.

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Chapter 2 is set three years after James began living with his aunts, on a day when rather peculiar, very peculiar, and fantastically peculiar moments occur. James' two aunts watch him while he chops wood in the heat, each of them bragging loudly about her own beauty, complimenting her own eyes, hair, clothing, and other bodily features through rhyme. While James chops wood, he thinks of other children in the world and envies their happiness. At one moment, he is overwhelmed by his thoughts and begins to cry. When his aunts begin to yell at him, he begs them to set aside a day to take him to the beach. Aunt Spiker and Aunt Sponge threaten to beat James, and James runs away to a corner of the garden and begins to cry hysterically.

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In Chapter 3, James is crying in the corner of the garden when an Old Man appears from behind the bushes. This is a small Old Man who rests on his cane for support, and he calls James to come closer to him. When James moves towards the man, the old man reveals a small white paper bag filled with tiny green things that resemble crystals, each one about the size of a grain of rice. Soon James sees that the tiny green crystals are moving, and the Old Man tells James that these little items had "more power and magic...than the rest of the world put together." When James asks what the items are, the Old Man responds that they are "crocodile tongues boiled up in the skull of a dead witch for 20 days with the eyeballs of a lizard." After this brief yet confusing explanation, the Old Man gives the bag to James and tells James that the items are now his.

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In Chapter 4, the Old Man gives James special instructions for using the moving magic crystals. James must pour the little green things into a large jug of water and then add 10 of his own hairs to the mixture, one by one. As soon as the mixture begins to boil, James is to drink it down, and then magical things will begin to happen to him. He will no longer be miserable. The Old Man gives one last instruction before he leaves James - don't let the green things escape, or else they will work their magic upon somebody else instead!

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In Chapter 5, James excitedly runs off with the bag and makes a plan to create the mixture in the kitchen, in secret from his aunts. In his rush to get to the kitchen unnoticed, James trips on the roots of the ancient peach tree and the bag breaks. The tiny green things land on the ground and, as James frantically tries to gather the crystals, they sink into the ground and burrow into the soil. Devastated, James feels that all of his luck has been lost. His aunts come over to him and yell at him for being lazy, threatening to punish him by making him sleep in the water bucket to the well. He is instructed to continue chopping wood immediately, when all of a sudden he hears a shout that makes him stop and turn.

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At the beginning of Chapter 6, Aunt Spiker shouts to Aunt Sponge that a peach is growing on the once-barren tree. Aunt Sponge is shocked. James puts down his yard tools and walks up to his aunts; he has a feeling that something peculiar is going to happen. His aunts want to eat the peach, splitting it half-and-half, and they ask James to climb the tree to the tallest branch and get the peach down for them. When James reaches the top, his aunts yell at him to not eat any of the peach before he brings it down. But just before James reaches the peach, both of his aunts shout out that the peach is growing! In no time at all, it has doubled in size.

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In Chapter 7, the peach begins to grow uncontrollably. James and his aunts all watch as it continues to grow, assuming that the now-huge fruit will break the branch and fall to the ground. The branch does not break, however, and the peach soon settles on the ground and stops growing. At this point it is the size of a small house and is perfectly ripe. James's aunts look at the beautiful peach and (at Aunt Spiker's prompting) decide there is money to be made from this strange sight.

 

In Chapter 8, news of a peach as large as a house spreads across the nation, and Aunt Sponge and Aunt Spiker capitalize on the opportunity for publicity. They build a fence around the peach and charge 1 shilling per person for admission to see the peach, double if the visitor brings along a camera. While his aunts are busy making money, James is forced to stay inside his room and simply watch. Aunt Spiker and Aunt Sponge even become so busy counting their money that they forget to feed James. What makes the situation even sadder is that James desperately wants to go outside and play with the children who come to see the peach, since he hadn't met another child in several years.

 

Chapter 9 opens with James staring out at the peach in the middle of the night, longing to go out and touch it. He makes his way outside, where the shadows dance around him, and he is tense with fear as he stares at the peach. It looks like a tremendous silver ball, and James, despite his apprehension, walks towards it, ready to finally feel it. As he walks, James has a feeling that something spectacular is about to happen. The garden is alive with magic. As he places his hand and cheek on the soft fuzz of the peach, he looks down at the ground - there is a hole in the peach.

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James excitedly decides to explore the hole in Chapter 10 - but as he crawls in, he realizes this isn't a hole, but a tunnel! He begins to crawl along the soggy floor of the peach, inhaling the sweet smell of the fruit and tasting the juice as it falls from the ceiling of the tunnel. He crawls uphill, and seems to be approaching the center of the fruit. Suddenly he hits his head on a hard surface, which appears to be a wall at first. Upon closer examination, James realizes the grooved and waxy "wall" is actually the pit of the peach. Within the pit is a small door, and James opens it and crawls in. He is greeted by voices: "Look who's here! We've been waiting for you!" As he looks up and identifies the speakers, he is horrified and turns white. He tries to turn around and exit, but nothing but a solid brown wall is now behind him.

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In Chapter 11, James becomes acquainted with the creatures who had startled him at the end of Chapter 10. As he gazes around the interior of the peach, he sees a collection of insects: a grasshopper, spider, ladybug, centipede, and an earthworm. But were they really insects? James had thought insects were small, but these insects are very large - the grasshopper alone is as large as a large dog. As James regards them with fright, the creatures begin to discuss how hungry they are. They seem to focus ominously on James. They ask him if he is hungry, but he is too scared to offer a response. Suddenly, the creatures realize that James is afraid that they want to eat him. They laugh uproariously at the idea, assuring James that he is one of them now and that they've been waiting for him all day.

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In Chapter 12, James lends assistance to one of his new acquaintances; he helps the Centipede remove his boots before bed. The Centipede tells James that they will have to remove 100 shoes, but the others creatures, particularly the Earthworm, insist that the Centipede is constantly lying and only has 42 legs. As James helps remove the boots, he listens to the creatures bicker with one another. The Earthworm believes that no legs is a superior mode of life, while the Centipede insists that more legs are better; this one creature is also very proud of the fact that he is a pest. The Old-Green-Grasshopper tries to moderate the conversation, telling the others to calm down. Throughout all of this, James concludes he likes the good-humored Centipede, despite the Centipede's demanding ways; it is very nice to hear laughter after so many years of life under Aunt Sponge and Aunt Spiker. The Centipede then begins to tell James the story of how he became such a marvelously large centipede. He was going about his business in the garden under the peach tree, when suddenly a little green thing wiggled past his nose. Other insects - the Ladybug, Miss Spider, and the Old-Green-Grasshopper - also claim that they saw the same thing, and James exclaims that he knows what it was: the magic items. The creatures all say that they ate one, two, or three of the green creatures, and as they are about to complete the story, the Old-Green-Grasshopper urges them to get ready for bed because they have a long day ahead of them the next day.

 

In Chapter 13, Miss Spider gets to work preparing beds for everyone, which she accomplishes by spinning hammocks out of silk. James continues to work on Centipede's boots for two hours, and by the time he finishes Centipede has fallen asleep. James must wake the Centipede up so that he, the Centipede, can go to bed in his hammock. As James settles down in his comfortable sleeping position, the Centipede yells "Lights out!" to an unnamed person. James looks up and sees that The Centipede is addressing a Glow-worm, who is attached to the ceiling and has fallen asleep with her light on. Centipede rudely wakes her, and she turns off her light. As James prepares to go to bed, he realizes how much he likes his new friends. They are not as terrible as they had once looked, and they are actually incredibly kind and helpful.

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James wakes to a series of shouts proclaiming, "We're off!" at the start of Chapter 14. All of the creatures are moving around excitedly, and it seems as if there were an earthquake taking place. The Ladybug, a kind and gentle creature, explains to James that they are about to depart forever from the hillside where Aunt Sponge and Aunt Spiker live. The Centipede is even on top of the peach, using his sharp jaws to cut away the stem attaching the peach to the tree in the garden - even though no one knows exactly where the peach will go once released. Soon the peach is free and begins rolling down the hill, and the furniture, creatures, and James are all slammed against the walls by the momentum.

 

In Chapter 15, Aunt Sponge and Aunt Spiker are preparing for the next batch of tourists who have come to see the peach. After a brief discussion about where James went last night - both of them hoping that James was seriously injured during his night away from the house - the two women discuss how they are going to make a fortune. Suddenly they hear an alarming sound. It's the peach! It is rolling down the hill, quickly gathering speed, and the two aunts try desperately to get out of its way. Yet Aunt Sponge trips over the box she brought to collect the money, and Aunt Spiker trips over Aunt Sponge. Before they can get up, the peach rolls over them and with a crunch, both of James' aunts are laid out lifeless on the hillside.

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In Chapter 16, the peach continues to gather speed and momentum. It rushes straight past the people walking up the hill to see it, and everyone begins diving left or right in order to avoid getting squashed. It rolls across fields, roads, and towns, taking down anything that is in its way. At one point it breaks through a chocolate factory, leaving two holes in the building, and chocolate flows out of the factory and into the streets. Children began swimming in the chocolate and try to eat it, but the peach just keeps rolling. Suddenly the peach approaches the sea where James used to live, the sea that he wanted to visit the other day; there is a line of steep, famous cliffs facing the shore. The sea is described as very dangerous, and it appears that there is no hope for James and his new friends as they plummet off a cliff and into the water. The peach is submerged but then floats back to the top, sitting easily on the surface of the sea.

 

James, Centipede, Earthworm, Miss Spider, Ladybug, Glow-worm, and Old Green Grasshopper are recovering from the rough-and-tumble trip from the garden to the sea as Chapter 17 opens. They had been flung back and forth, and now they are tangled up with one another. They begin to sort everything out with the help of the Glow-worm's light, then start to speculate about where they could have landed. They eventually decide to go to the top of the peach and look around, a much safer alternative than going out the side entrance, since they don't know where they are. Miss Spider busies herself with weaving a ladder from the floor of the peach to the roof, and the others assist Centipede as he puts on his 42 shoes. When the shoes are on and the ladder is finished, they all climb to the roof, excited about what they might see next.

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In Chapter 18, everyone is shocked to find that they are in the middle of the sea. The Earthworm proclaims that they are all finished, destined to drown, at least until James explains the peach is actually floating. Although the Old-Green-Grasshopper maintains that everything will be fine in the end, Earthworm tries to upset the others by insisting they are still in a bad situation - they have no food. James interrupts again by explaining that the entire ship is food - they can gradually eat the peach, and they will have no problem surviving for weeks and weeks without destroying the peach as a vessel. Earthworm, very flustered, now says that the problem is that there is no problem! The others joke with him that he is always looking for something to complain about.

James and the creatures all begin to eat the peach, and find that it is simply delicious. Many of them say that it is the most delicious thing they've ever tasted, and Centipede bursts into song. He details all of the delicious things that he has eaten in the course of his life, concluding that this peach is the best of them all. Everyone is joyous and content.