The story starts in the first week of August when three things happen on the same day. The first is the arrival of Mae Tuck in the Treegap woods, which were owned by the Foster family. The second is the arrival of the man in the yellow suit at the Foster's home. The third is when Winnie Foster decides to run away; she is tired of her family's strictness and wants to live in complete freedom. She decides to run away at dawn the next day. Her pondering is interrupted by the arrival of the man in the yellow suit, who asks if her family owns these woods.
On the day Winnie ran away, Mae Tuck goes to meet her two sons, Jesse and Miles, whom she hasn't seen for about ten years. As Winnie goes deeper into the woods, she finds a spring with a boy beside it, drinking from it. The boy's name is Jesse. Winnie asks to drink some water but Jesse refuses to let her and also refuses to tell her why. Jesse is relieved when his mother, Mae, and his brother, Miles, arrive. They are also shocked to see Winnie and decide to take her with them, placing them on their house and dashing through the woods. On the way, the man in the yellow suit spots them with Winnie.
The family tells her their story: they passed by Treegap eighty-seven years ago, saw the spring, and drank from it. They later discovered that unlike everyone else, they did not age and could not be killed or harmed. Miles' wife left him because she thought he had made a pact with the devil, and all of the Tucks' friends thought they were witches and practiced black magic. The Tucks then had to move from place to place in order to avoid suspicion. The Tucks told Winnie that they had not meant to kidnap her, but they had no other choice because they needed her to understand why the spring was so dangerous. Winnie, only ten years old, wants to go home, but she also feels loved by the Tucks. Unbeknownst to the Tucks or Winnie, the man in the yellow suit overhears this conversation.
In Treegap, the man with the yellow suit visits the Fosters and tells them that he knows who kidnapped their daughter and where she is. In return, he wants them to give him ownership of the woods. The Foster family has no choice but to agree.
The man in the yellow suit finds the Tucks and tells them his plans: now that he owns the woods, he will sell the water for a fortune. He tells them that his grandmother had known a family that never seemed to age, and a woman who had left her husband after he seemed not to age. The husband was Miles, and ever since then, the man in the yellow suit has been determined to find the family. The man grabs Winnie by force and threatens to force her to drink the spring water, and Mae has no choice but to hit him with a gun, knocking him unconscious.
Unfortunately, the constable witnesses this incident and takes Mae into custody. Mae will be locked up and Winnie will be returned home. The constable decides that if the man in the yellow suit dies, Mae is to be killed by hanging at the gallows. The Tucks panic at this thought (because Mae cannot die, and this might cause her great pain as well as expose their secret to the whole world), but Winnie says that everything will be alright.
Sure enough, the man in the yellow suit dies. When Winnie returns home, Jesse goes to meet her and says goodbye to her. He also gives her a bottle of the spring water and asks her to drink it when she turns seventeen like him so that they can marry.
Miles removes the window of Mae's cell using his carpentry skills, and Winnie helps by taking Mae's place so the constable will not notice Mae's disappearance. Winnie covers herself up under the blanket and pretends she is Mae sleeping. This gives the Tucks enough time to get far away from Treegap. The next morning, Winnie is discovered, but the constable can't punish her because she is too young. Winnie pours the spring water that Jesse gave her on a toad, reasoning that she knows where the spring is and can always get more.
Many years later, the Tucks come back to Treegap, but they are shocked to find that the woods are gone; a fire destroyed them. The Tucks visit Winnie's grave, and find out she lived a normal life as a wife and mother. The family feels sorry for Jesse, who had high expectations to marry Winnie, but Angus Tuck feels she made the right choice by choosing not to drink the water.