Ten-year-old Winnie Foster is from a wealthy family and lives in the small town of Treegap in the late 1800s. Across the road from her house, there is a small forest known as “the wood.” One night, while Winnie is catching fireflies in her front yard, a strange, old man in a yellow suit arrives at the Foster home. He speaks to Winnie from the other side of the fence. He is polite and says that he is looking for someone. Winnie’s grandmother comes out of the house and the three hear strange music coming from the wood. Winnie’s grandmother does not answer the stranger’s questions and takes Winnie inside the house.

Winnie becomes tired of her mother and grandmother constantly watching over her. She thinks about running away but does not want to be alone. Instead, she decides to go into the wood across from her house, something she has never done before. Inside the wood, Winnie meets Jesse Tuck. He is drinking from a natural spring beside a large ash tree. He is seventeen, and Winnie thinks that he is beautiful, despite his having ragged clothing and no shoes. Winnie asks about the spring and Jesse tells her that she cannot drink from it. While Jesse and Winnie are talking, Jesse’s mother, Mae, arrives. 

Mae is worried that Winnie has found out about the spring. She takes Winnie and puts her on the back of the Tuck family horse. Jesse, Mae, and Jesse’s brother Miles take Winnie away from the town. On their way out, Winnie sees the stranger in the yellow suit but does not yell for help. 

The three Tucks take Winnie to their house, twenty miles outside of Treegap. Along the way, the Tucks explain to Winnie that anyone who drinks from the spring will stop aging and cannot die. They do not know that the stranger in the yellow suit has followed them and overheard them talking. The Tucks tell Winnie that they will take her home the next day, after she has talked to Angus Tuck, Mae’s husband. 

Winnie is homesick but likes how the Tucks care for her. Angus explains that no one can know about the spring, because then everyone would want to live forever. This would be a disaster. Angus tells Winnie that it is terrible never to age or change. The Tucks cannot have friends, and Miles, who was married, was abandoned by his wife and kids. Anyone who notices that the Tuck family does not age thinks that it is unnatural. 

Jesse tells Winnie that she should wait until she is seventeen and then drink from the spring. Then, they can both be seventeen forever and travel the world. Winnie is still attracted to Jesse and this idea excites her. While Winnie is staying at the Tucks’ house, the stranger in the yellow suit steals the Tucks’ horse and rides back to Treegap. He tells the Fosters that he can save Winnie, but they must give him ownership of the wood across from their house. The Fosters agree.

The stranger and Treegap’s constable ride back to the Tucks’ house outside of town. The stranger is impatient, so he rides ahead. The stranger arrives at the Tucks’ house while they are having breakfast. He tells them that he had heard about a family that never aged and has spent his life trying to find them. He tells the Tucks that he now owns the wood. He says that he will take Winnie and make her drink from the spring, so that he can use her in demonstrations to prove the powers of the spring. The stranger plans to sell the spring water. 

The Tucks are outraged. Mae hits the stranger in the head and kills him. The constable arrives in time to see Mae hit the stranger and arrests her. He takes Mae and Winnie back to Treegap. Winnie tries to explain that she was not kidnapped and wanted to go with the Tucks. Once Winnie is home, Jesse visits and gives her a bottle of spring water for when she turns seventeen. Jesse tells Winnie that he and his brother are going to break Mae out of jail. Winnie wants to help. Jesse and Miles remove the bars from the window of the jail and help Mae out of her cell. Winnie lies down on Mae’s cot under a blanket so that the constable does not know Mae escaped until morning. The Tucks escape and Winnie never sees them again.

Weeks later, in her front yard, Winnie saves a toad from a dog. She has seen the toad before. She pours the bottle of spring water over the toad, believing that it will protect him. Seventy years later, Angus and Mae travel to Treegap. It is very different and the wood is gone. They find out that a lightning strike burned the wood down. Angus goes to the cemetery and finds the grave of Winnie Foster. He is sad that she is gone, but happy that she never drank from the spring. On the way out of town, they see Winnie’s toad. It is still alive.