In 1880, ten-year-old Winifred "Winnie" Foster, who lives at the edge of the American village of Treegap, thinks about running away from her overbearing family. That evening, a man in a yellow suit approaches the Foster home, looking for information. Meanwhile, Winnie, the man, and Winnie's grandmother hear a music box playing deep in the woods near the Fosters' house.
The next morning, Winnie explores the wood. While the Fosters own the area, they never enter it. She sees a young man, Jesse Tuck, drink from a small spring. When she asks his age, he first says he's 104 years old, then changes his answer to seventeen. Winnie wants to drink from the spring, but Jesse stops her. When she mentions her father, Jesse becomes scared she will tell him about the spring. Jesse's mother Mae and brother Miles arrive. They seize Winnie, taking her to their home and pleading with her not to be scared. On the road, they pass the man in the yellow suit.
The Tucks explain that, 87 years ago, they had passed through the wood while looking for land to farm. They drank from the spring. After twenty years, the Tucks realized they were not aging. Miles's wife left him, taking their son and daughter. Forced to leave their farm, the Tucks returned in the direction of Treegap. After seeing that the clearing around the spring had not changed in twenty years, the Tucks realized that the spring had made them immortal.
Winnie quickly grows fond of the Tucks. Angus, patriarch of the family, explains that he wants to grow old but can't. He asks her to keep the Tucks' secret, saying that if others found out about the spring, they would drink from it and later regret their immortality. That evening, Jesse proposes that after Winnie turns seventeen, she drink from the spring and live eternally with him.
The man in the yellow suit steals the Tucks' horse and rides to the Fosters' home. He tells them that the Tucks have kidnapped Winnie and promises to rescue her in exchange for the Fosters' wood. He directs the constable to the Tucks' home, then rides ahead to meet the Tucks alone. He tells the Tucks that his grandmother had a friend who left her husband, taking their son and daughter, because the husband had not aged in twenty years. The ageless husband's mother had owned a music box. The man in the yellow suit had learned the music box's tune from his grandmother, who had learned it from her friend. Two nights ago, the man had heard the same tune coming from the Fosters' wood, where Mae had played it on her music box. He had followed the Tucks and eavesdropped to hear their story. Now, with legal possession of the wood and spring, he plans to sell the spring water to "people who deserve it. And it will be very, very expensive."
The man in the yellow suit offers to pay the Tucks to publicly demonstrate their invincibility. Angus angrily refuses. The man drags Winnie outside. He announces that after she drinks the water, demonstrations with her will do just as well. Mae grabs Angus' shotgun by the barrel and, despite Miles trying to stop her, swings the gun at the man with the yellow suit; just as the constable arrives, Mae lands what will prove to be a fatal blow to the man's head.
Winnie tells the constable that the Tucks are her friends, not her kidnappers. The constable takes Winnie home and locks Mae in the village jail; when word reaches the jail that the man in the yellow suit has succumbed to his injury, Mae is scheduled to be hanged for killing him. The Tucks and Winnie realize that attempting to execute her will reveal the Tucks' secret.
The following evening, Jesse visits Winnie. He explains that Miles has a plan to break Mae out of jail. He gives her a bottle of water from the spring and asks her to drink it when she turns seventeen. Winnie volunteers to help Mae escape. At midnight, Winnie and the Tucks go to jail. Miles pries the window out of Mae's cell, and Winnie exchanges places with Mae. In the dark, the constable mistakes Winnie for Mae. He does not realize until morning that Mae has escaped, and by then the Tucks are gone.
Infuriated, the constable yells at Winnie for committing a crime, and that if she was older, he would've kept her there. Winnie is not given a direct punishment as she is too young to be punished by law.
Two weeks pass. Winnie sees a toad threatened by a dog. She snatches up the toad and pours the water from Jesse's bottle over it.
Decades later in 1950, Mae and Angus Tuck return to Treegap. At a diner, the couple learns that the wood was struck by lightning and burned in 1947. It was bulldozed, and now a gas station stands on the site. In town, Angus finds Winnie's grave in a local cemetery revealing that she died in 1948 at the age of 78 and was married with grandchildren. He feels proud. As they leave the town, they see a toad squatting on the road. Angus moves it to the side of the road, commenting that it must think it's going to live forever. The family moves on forever from Treegap.
