The narrator explains that the first week of August is the highest point of the year's cycle and that during this time, people do things they regret later. The world, the narrator suggests, is like a giant wheel, with spokes connected in the center by a hub that, in this case, is a small wood owned by the Foster family. On the outside of this hub, connected by the wood, are the Tuck family, ten-year-old Winnie Foster, and the man in the yellow suit. The wood is a strange place and it makes people want to avoid it. The narrator insists that this is wise, as if a person were to go into the wood, they'd discover a huge ash tree and a spring coming from its roots. Discovering this stream would be a disaster.
On the first day of the first week of August, Mae Tuck wakes up, excited to go meet her sons in Treegap. Her husband, Angus, isn't excited and assures Mae that he'll be fine while she's gone, as nothing can happen to him. Mae ignores this and the narrator explains that nothing can happen to Angus, Mae, or their sons, Jesse and Miles: they're all immortal. At noon on this day, Winnie sits inside her family's fence and tells a toad on the other side that she'd like to run away so she can be independent and do something important. At sunset, the man in the yellow suit appears at the Fosters' gate and asks Winnie if she knows everyone in Treegap. The man makes Winnie suspicious, but she speaks to him and suggests that he talk with her father. Winnie's Granny rescues her from the conversation, though they all hear tinkling music that makes Granny pause excitedly. She tells Winnie that this is the elf music she hears every now and again and leads Winnie inside. The man in the yellow suit walks away, whistling the tune from the "elf music."
By the next morning, Winnie has decided that she's too afraid to be alone to run away, but she does decide to walk into her family's wood and explore. The wood is surprisingly pleasant, especially when she comes across a handsome young man who introduces himself as Jesse Tuck. Winnie is immediately smitten and asks Jesse if the water she saw him drinking is good to drink, as she's thirsty. He is trying to keep her from drinking the water when when Mae and Miles show up. They immediately throw Winnie onto their horse and lead her away quickly. They pass the man in the yellow suit on the far side of the wood and ask Winnie to not scream or be afraid as they go. Finally, they reach a stream on the other side of a meadow, offer Winnie Mae's music box to look at (which Winnie learns is the source of the "elf music") and tell Winnie a fantastical story: 87 years ago, they drank from that stream in the wood and since then, they haven't aged at all. Miles's wife left with their children after 20 years of marriage, believing that Miles sold his soul to the devil, and the Tucks eventually figured out that the water was the source of their immortality.
Winnie isn't convinced, but the Tucks are clearly relieved to have told someone. Jesse says that being immortal is fantastic, but Miles suggests that Jesse should take things more seriously. Mae asks Winnie to help them keep their secret and says that they need to take her home with them so she can understand, but they'll bring her back the next day. Winnie decides to agree and feels as though anything is possible. The world feels wide and wonderful, and she and the Tucks don't notice that the man in the yellow suit was also listening to the story.
It takes hours to reach the Tucks' homestead. Jesse and Miles jump straight into the pond, while Angus greets Winnie with a smile. Winnie is shocked to see how disorderly the Tucks' home is: it's dusty and organized haphazardly, but it feels homey and comforting. Mae explains that Jesse and Miles spend their time away, working when they can, and says that Jesse's eternal youth means he can't settle down. She says that she and Angus will have to move soon so people don't begin to suspect anything about them, which Winnie thinks is sad. Mae suggests that life needs to be lived, no matter how long it is. Next, the Tucks and Winnie have supper. Winnie is shocked to eat in the parlor without napkins, and soon she begins to think that eating is a private, intimate activity that she shouldn't be doing with her kidnappers. She announces that she wants to go home, and Angus says that he'll take Winnie out on the pond to talk. They discuss how the man in the yellow suit saw the Tucks leading Winnie away, and this thought gives Winnie confidence--she believes the man will tell her father and help her get home.
Winnie boldly gets in the boat with Angus. He softly tells her that in the pond, all around them, creatures are growing and changing. He tells her that the water in the pond will eventually reach the ocean and then return to the pond as rain. He allows the boat to get stuck in some roots on the downstream side of the pond and says that the Tucks are like the boat; they're stuck and they can't continue to grow and change. Winnie realizes that she's going to die, which shocks her and makes her suddenly angry. Angus says that this feeling is normal, but Winnie nonetheless needs to die someday, as living without dying is barely living at all. He says that if people drank the water from the spring, they'd never understand what they're giving up. Miles interrupts by yelling that someone stole the horse.
The man in the yellow suit arrives at Winnie's parents' house a while later on the Tucks' horse. He explains that he knows where Winnie is and makes Winnie's father agree to give him the wood in exchange for getting Winnie home safely. Then, the man wakes up the constable and the two start out for the Tucks' homestead, the man in the yellow suit riding on ahead.
Mae makes the sofa into a bed for Winnie, who's not happy about going to bed in a stranger's house, without her nightgown or bedtime routine. As Winnie starts to fall asleep, Mae tiptoes out of the bedroom to apologize and says she wishes that Winnie were theirs. A bit later, Angus comes to check on Winnie and offers to sit with her until she falls asleep. He kisses her on the cheek before returning to bed. This makes Winnie feel cared for and she thinks that they're not actually criminals. Finally, Jesse creeps down and suggests that Winnie drink the water when she turns seventeen so she can see the world with him.
Winnie wakes up early in the morning. Miles gets up too and invites her to go fishing with him. As they step into the boat, Winnie thinks that the Tucks are her friends and she loves them. Miles explains that his daughter, Anna, was a lot like Winnie. He says that it would've been unnatural for his children and wife to drink the water, though he thought about it. Winnie remarks that it'd be nice if things never had to die, but Miles points out that if nothing died, there'd be no room for new life in the world. Miles takes over fishing as Winnie decides that Miles is right. Miles says he wants to do something useful to earn his place in the world, though he's not sure what to do. Just then, he catches a fish and pulls it up into the boat. Winnie wants to cry and asks Miles to put it back. He does, but he also reminds her that people sometimes need to eat meat and kill things in order to live.
At breakfast, Winnie is excited to go home but also decides that the Tucks really are her friends. She thinks about the possibility of drinking from the stream as she studies Miles, Jesse, and then Angus, who looks sad. They hear a knock on the door and open it to find the man in the yellow suit. He explains that he heard the story of a family that never ages from his grandmother, and using the melody from the music box, tracked the Tucks down. He wants to bottle and sell the stream's water and sell it to "deserving" people, and he'd like the Tucks' help in marketing the water. When they refuse, the man grabs Winnie and says he'll make her drink to use her instead. At this, Mae hits the man with the butt of her rifle, knocking him out immediately. The constable sees this as he arrives. He's shocked when Winnie says that the Tucks didn't kidnap her, and he insists that he needs to take Winnie home and put Mae in jail. If the man dies, Mae will be put to death by hanging. As the constable starts to ride away with Winnie, Winnie tells Angus that it'll be all right. She knows that Mae cannot die even if she goes to the gallows.
That night back in her own home, Winnie sits in a small rocking chair and thinks about what happened. She thinks about her experiences with the Tucks and decides that those experiences are, satisfyingly, all her own. She wonders if the Tucks' story is true and thinks that if it is, the man in the yellow suit has to die. Just then, the constable knocks on the door and Winnie hears him say that the man died. Winnie remembers feeling awful for killing a wasp once and wonders if Mae feels bad for killing the man. She vows to do something to keep Mae from going to the gallows.
Winnie goes outside after breakfast the next day. She notices the toad and asks Granny to help her give the toad some water, but the toad disappears in the time it takes Winnie to fetch a bowl. A bit later, Jesse appears and explains that they're going to break Mae out of jail later. Winnie says that she wants to help by taking Mae's place in the cell so that the Tucks have more time. She also accepts Jesse's bottle of spring water, which he tells her to drink when she's seventeen. Winnie spends the rest of the day waiting for midnight. She wonders if her parents will ever trust her again and is shocked when they don't wake up to stop her sneaking out. As Jesse leads Winnie to the jailhouse, a thunderstorm starts. Miles pulls out the nails of the window and, with a clap of thunder, finally pulls the metal window frame right out. Mae crawls out and after each of the Tucks hugs and kisses Winnie, they boost her inside and refit the window. In the morning, Winnie reveals herself to the constable. She's too young to be charged, but her parents confine her to the yard indefinitely. Winnie tells her mother that she did it because she loves the Tucks and they're her friends, and her family accepts this. Fortunately for Winnie, her rule-breaking makes her more interesting to other children in town. Winnie notices the toad, which is being harassed by a big dog. She grabs the toad, brings it inside the fence, and fetches the water from Jesse. She pours it on the toad and reasons that if she wants to join Jesse later on, she can go drink from the stream.
Years later, Angus and Mae ride into Treegap. The town is barely recognizable; it has pavement and a number of shops, while Winnie's cottage is gone. They also can't find the wood. A man at a diner tells them that the wood was struck by lightning a few years ago, burned down, and it was bulldozed. While Mae goes shopping, Angus goes to the graveyard and finds Winnie's grave. He sees that she's been gone two years and was a wife and mother. He cries and salutes her. As he and Mae leave town, Angus saves a toad from being run over.