This story begins on a typical day for the Stone family- Mama is suffering from a nasty hangover, and Brian is trying to think of a way to keep out of sight and protect his little brother Brandon. Things quickly become anything but normal, however, when Brian finds a silver necklace in one of his grandfather's old Army trunks in the attic and discovers that it can do magic. Excited by this possibility and yet fearful of letting his mother find out about it, Brian decides to slip outside and experiment with the amulet for a few minutes while Brandon is still asleep. He soon discovers that the amulet has incredible powers, but when he returns to the house in half an hour or so, he finds that Brandon has woken up already and that Mama has blacked his eye. Brian uses his new-found amulet to heal the child, filled with guilt over leaving him alone.
Over the next few days, Brian discovers all sorts of things about his powers. He turns gravel into gold nuggets which he sells for cash, and changes their dilapidated house into a gleaming showplace. He makes friends with several of the snobs who looked down on him before, and learns not to fear his mother so much. But disaster strikes when he discovers that the amulet only works for seven days, and that any living thing he changed with it will die. That includes Brandon, and Brian in desperation snaps the chain of his necklace, thereby giving up all the things he's done with it. But Brandon is still deathly sick, and Brian discovers that he has no choice but to follow the amulet to the Heart of the World and drink from the Fountain there if he wants to save him.
He leaves town on his dirt bike, following the compass which he found inside the amulet. He tells no one about his journey. He stops for lunch the next day in Falls Chapel, where he encounters Sadie Jones, an old lady who claims to have drunk from the Fountain herself many years ago and who invites him to come to her room at the nursing home the next day, when they can speak privately. Brian agrees, and spends the night at a bed-and-breakfast nearby. As it turns out, while staying at the bed and breakfast he runs into Rachel McCray, a girl he knows from school. She agrees to keep his secret, if she can come along on the journey to the Fountain. Rachel suffers from Batten's Disease, a fatal genetic disorder which will kill her before she turns 20 years old unless she discovers a miracle.
But in the meantime Brian's mother discovers him missing and calls the police, who find him at the bed and breakfast. Brian escapes and meets up with Rachel early the next morning, and together they visit Sadie Jones before leaving town. Miss Sadie tells him some of her experience with the Fountain, and warns him that there will be a price to pay for the power he seeks.
Brian and Rachel leave town in her sister's car, traveling north into the heart of the Ozarks. They eventually have to abandon the car when the compass leads them onto a dirt road which is blocked by a fallen tree, and they find themselves forced to walk through rugged hills for two days. On the second night they sleep in an abandoned hunters' cabin, which catches on fire during the night and nearly kills them both. Brian has to rush back inside to rescue Rachel, who slept on the top bunk bed and suffered worse from the smoke than he did.
The next day they find themselves at the mouth of a cave, and for the next two days they have to crawl underground with nothing to eat but pears. At one point they nearly drown in an underground river, and Rachel loses her medication and starts having seizures. They are attacked by bats and their flashlight goes dead. Brian has no choice but to carry her on his back. Before long he is exhausted, broken-hearted, and nearly in despair.
Finally, they reach the Heart of the World and drink from the Fountain. This heals all their hurts and also gives them the power to heal every wrong thing in the world. They escape by swimming out through a flooded cavern into the bottom of a sinkhole, then climbing the walls. They hitchhike into Jasper, where Brian calls one of his friends to give them a ride home.
They stop at the hospital where Brandon is a patient, only to discover that he has died before they could get there. In his misery, Brian forgets how much power he now has. But Rachel remembers, and manages to convince the morgue-keeper to let them see Brandon's body. Together they lay hands on the child and pray, and breath life back into his body, causing a minor sensation at the hospital.
Brian also finds that he is able to erase his mother's alcohol addiction, and that Brandon's near-death and his own disappearance have provided the moral shock necessary to get her and his father to make a real effort at change.
Over the next few months, Brian and Rachel use their powers to turn the area into a beautiful but subtle memorial of what could have been if the world had never fallen. But they also find that they must pay the price for this, and that within a few years they'll have to leave and never return so that no one will notice that they no longer age.
Near the end, Brian discovers that Brandon has had a vision of the Heart of the World before he woke up in the hospital, and that he seems convinced that someday he'll be expected to go find it himself, for a reason he's not sure of yet. But his memory is vague, and Brian is never able to find out any more.
A few years later, Brian and Rachel get married and leave Falls Chapel together, to travel the world and plant blessings in other places.