Ten-year-old Winnie Foster is tired of her family and is thinking of running away from her rural hometown of Treegap. One day, while in a wooded area her family owns, she sees a boy about the age of 17 drinking from a spring. He tells her that his name is Jesse Tuck, and he refuses to let her drink the water. Soon after, his brother, Miles, and mother, Mae, take her away with them and explain what is happening and why they did what they did. All the while, they are being pursued by a man in a yellow suit who the day before had approached the Fosters asking questions about their land.
The Tucks explain to Winnie that the spring is a magical one that grants eternal life to anyone who drinks its water and that they discovered its effects by accident after heading to the Treegap area to try and build a new life for themselves. In the process, Miles has had to deal with his wife leaving him and taking their children with her and they have been living in seclusion outside of Treegap for years, reuniting every ten years and drinking from the spring. Winnie grows particularly fond of Jesse and his father, Angus, who teaches her about the life cycle.
Meanwhile, the man in the yellow suit has been pursuing the Tucks and once he discovers Winnie having been taken by them and overhears their whole conversation, he steals their horse, and rides her back to the Foster homestead. Once he informs them of Winnie's whereabouts, they dispatch him to return her along with the local constable. However, he breaks away and rides ahead of the constable, as he has an ulterior motive for finding her.
As he arrives at the Tucks' farm, the man in the yellow suit informs them that he has been searching for them for years, with Miles' wife and children having come to live with his family when he was a boy. This was when he first heard rumors of their secret, and he intensified his search within the previous six months. He then informs the angry family that he told the Fosters where Winnie was and that he has received a bounty in exchange for her safe return: the wooded area, and with it the spring.
The man in the yellow suit then further angers the Tucks when he tells them that he plans to gather the water from the spring and sell it to the public. When they angrily refuse his offer to be partners in the venture, desiring their privacy over the money, he declares he does not need their permission to sell the water and begins to take Winnie away. Before he does, he tells the Tucks that if they will not be his examples then she will. He says that a child would be a better example, and there is nothing they can do to stop him. An infuriated Mae, in a last-ditch effort to put an end to the confrontation, grabs her husband's shotgun and begins wielding it like a club. The man in the yellow suit tries to make a break for it, and Miles tries to get his mother to stop what she is doing, but neither are successful as Mae pistol-whips the man in the yellow suit. The stock of the gun strikes the man in the yellow suit in the back of the head with enough force that his skull is fractured on impact. This all happens just as the constable arrives, and Mae is arrested on the spot. Later that evening, the man in the yellow suit succumbs to his injury and Mae is condemned to hanging for murdering him.
Realizing that the secret will be revealed once Mae is hanged, her family and Winnie go to the jail and spring her from her cell so Winnie can take her place and the Tucks can safely get away. Although they are reunited, there is no more reason for them to be in Treegap as Mae is now a fugitive from justice. Before departing, Jesse gives Winnie a bottle of the special water so she might drink it when she turns 17 then come with them and marry him. Beforehand, she gives it consideration, and she decides not to and pours it onto a toad, as she thinks that if she changes her mind she can go back to the spring.
Many years later Mae and Angus return to Treegap to find that it has changed a great deal from when they were last there. The wooded area is gone, as well as their spring, and the town has become a typical suburban metropolis. While there, they happen to visit a cemetery where they discover what happened to Winnie. She went on to marry and have children, passing away in 1948. While there, they come across a toad near her grave, unaware that it is the same one that she had poured water on years before.