Near the little village of Treegap, there is a wood with an "otherworld[ly]" appearance, owned by the Fosters, who live in the "touch-me-not cottage" at its edge. There is no road running through the wood, so no one knows about the giant ash tree whose roots nearly conceal a fresh water spring at its center; this spring has the potential to create an "immense disaster."

One evening in August 1881, eleven-year-old Winnie Foster is chasing fireflies out in her yard, when a strange man dressed in a "jaunty yellow suit" stops by. The man says that he is looking for someone. He seems intrigued when Winnie informs him that the Fosters have lived in that place "as long as there've been any people [there]." Winnie's grandmother comes out, and when the man turns to her, a "tinkling little melody" emanates from the wood. Winnie suggests that the tune is coming from a music box, but her grandmother insists that it is being made by elves. When the old woman excitedly hustles her granddaughter into the house, the mysterious man, alone now, regards the wood thoughtfully, then disappears down the road, ominously whistling the haunting tune that faintly lingers in the darkness.

The next morning, Winnie is again out in the yard, railing against the stifling controls placed upon her by her family. She considers running away but, lacking the resolve, decides instead to venture at least into the adjoining wood, where she has never been allowed. As she walks through the luxurious foliage, Winnie comes upon an enormous tree in a clearing. Sitting by the tree is a handsome young man, to whom she "los[es] her heart at once." Winnie watches as the man sips from a small spring that rises from the ground, then she steps forward and asks him for a drink. The man, who introduces himself as Jesse Tuck, worriedly tries to convince Winnie that it would be "just terrible" if she were to taste the water. As Winnie points out that Jesse has just taken some himself, another young man and a big, "comfortable-looking" woman appear, leading an old horse. The pair are Jesse's older brother Miles, and his mother, Mae Tuck. When Mae sees Winnie there, she says resignedly:

Well, boys...here it is. The worst is happening at last.

Before Winnie even has time to think, she is lifted onto the horse and is transported through the wood, while Mae, Miles, and Jesse run alongside, pleading with her not to be scared. At the edge of the wood, they encounter the man in the yellow suit, but Winnie is so bewildered, she does not call out for help. Awhile later, Mae decides it is time to stop, and Winnie, understandably overwhelmed, begins to cry. Her obvious distress disturbs the Tucks greatly, and Mae distractedly reaches into her pocket, pulling out a little music box and winding the key. When the melody begins, Winnie is comforted. She recognizes it as the strain that she, her grandmother, and the man in the yellow suit had heard coming from the wood in Treegap the evening before.

As they have promised, the Tucks now tell Winnie the reason for their precipitous flight; it is a story beyond belief. Eighty-seven years earlier, the Tucks had been passing through the wood adjoining Treegap and had stopped at the spring to get a drink. Everyone in their party had had some of the water, even the horse, but, significantly, not the cat. In the following years, the Tucks had slowly become aware that something strange was happening to them. Jesse, for example, had fallen onto his head from a tree and was not hurt at all; the horse had been shot by some hunters, but the bullets had gone through him without leaving a mark. Although the cat had died since the family had left Treegap, the rest of them, in addition to apparently being indestructible, did not seem to be getting any older. The Tucks, then, had happened to pass through Treegap again, and had remembered the spring where they had once stopped to drink. The tree by the spring, on which Tuck had carved a "T," had not grown at all, and the "T" looked as if it had been carved yesterday.

The Tucks had concluded that the spring had somehow made them immortal. They at first had experienced a tremendous euphoria. After thinking about the situation, however, they had decided that it could be a very dangerous thing if word about the water's powers were spread. To their knowledge, Winnie is the only other person who knows about the magic spring, and the Tucks are kidnapping her to give themselves a chance to convince her of the importance of keeping their "big, dangerous secret." Although Winnie is not sure that she believes the Tucks' story, she senses that they mean her no harm. Jesse sings and dances during the remainder of the trip, making Winnie laugh, and everyone is having so much fun that they do not notice that the man in the yellow suit has been following them, and has heard the whole story.

At long last, the group arrives at the Tucks' cottage, a "plain, homely little house" nestled in the forest next to a pond. They are greeted at the door by Tuck, a big man with a "sad face and baggy trousers," who regards Winnie lovingly, and says that she is "the finest thing that's happened [to them] in...at least eighty years." Winnie, who is accustomed to scrupulous cleanliness in her own home, is at first taken aback by the amiable disorder of the Tucks' abode. She soon adjusts, however, and begins to appreciate the comfort of the "charm[ing]...disarray." Mae explains to Winnie that Miles and Jesse are away most of the time, living their separate lives, but every ten years, during the first week of August, they meet at the spring and come home together, so the Tucks can be "a family again for a little while."

After supper that evening, Tuck takes Winnie rowing out on the pond. It is almost sunset, and the bullfrogs are croaking as tiny insects "skitter" on the surface of the water. The scene is idyllic, and Tuck asks Winnie to observe the living things all around them. He explains that everything is part of a circle of life, "always growing and changing, and always moving on." Although immortality had seemed like an amazing gift at first, Tuck now realizes that he and his family have been shut out from the natural process of life. Because of this, they are not really living. His fondest wish is to be allowed once again to experience growth and change, even if it means that at the end of everything, they will die.

Tuck believes that if people knew about the magic water, they would instinctively do anything to get it, and by the time they found out what it really meant to be separated from the natural progression of life, it would be too late. This is why he is so intent on making Winnie see the importance of keeping the spring a secret. As Winnie, overcome by the import of all she has just heard, sits in stunned silence, Miles calls out from the house that someone has stolen the horse.

The Tucks are at a loss as to who would want to steal from them. Tuck in particular has "a bad feeling about the whole thing." Since there is nothing they can do about it, however, everyone goes to bed for the night. As Winnie lies awake trying to sort through all that has just happened, she receives a series of solicitous visits from the family. First, Mae comes by to make sure she is comfortable, then Tuck checks in, and tells Winnie how much it means to him to have a "natural, growing child in the house," before kissing her gently on the cheek. Finally, Jesse stops to talk. He suggests that when she is seventeen like he is, she should drink some of the magic water, and come to find them. Jesse says that they could get married then, and live life happily ever after, forever.

In the early morning, Miles, finding Winnie awake, invites her to go fishing. As they sit in the rowboat, Miles reveals that he once had had a wife and two children. He remembers his daughter especially; she would be eighty now, if indeed she is still alive. Winnie asks Miles why he had not taken his family to drink the magic water so that they too could be immortal, and Miles explains that by the time the Tucks had realized what had happened to them, it had been too late. His wife had been almost forty, and had left him and taken the children when she realized that, inexplicably, her husband was not getting older as he should. When the Tucks had finally figured out the secret of the magic water, Miles had considered looking for his family, but he would have been nearly the same age as his children, and everything would have been "so mixed up and peculiar, it just wouldn't have worked." Thinking about Miles' story, Winnie decides that the Tucks are right: it would be best if no one knew about the magic spring. She will keep their secret.

Meanwhile, the man in the yellow suit, who has stolen the Tucks' horse, has made his way back to Treegap. He proposes to the Fosters that he can tell them where their daughter is, in exchange for ownership of the wood—"a simple, clear-cut trade." When the Fosters, desperate, sign the deed to the land over to him, the sinister man goes to get the constable, and promises to return with Winnie. The constable, however, is a garrulous man and is not about to be hurried to his destination. Impatient to have the exchange completed, the man in the yellow suit arranges to go on ahead; the officer will meet him later at the Tucks' cottage.

At breakfast, the Tucks are discussing how they will get Winnie back home, now that they are without a horse. Mae surveys them all and comments about how nice it is to have everyone there together. Winnie, too, regards the Tucks with affection; they have become very dear to her, especially Jesse, and old Tuck himself.

The tranquility of the scene is abruptly broken by a knock at the door. It is the man in the yellow suit, who announces that he has come to take Winnie home. The man then proceeds to tell a story that is fraught with coincidence and quite extraordinary. As a child, he had been mesmerized by tales told by his grandmother, about a family which never got any older. The fantastic idea had become a lifelong obsession for him, and a few months previously, he had left his home to look for them, following the route they had purportedly taken so very long ago. The man's grandmother had heard about the odd family through a good friend who had married one of the sons, who is clearly Miles. The couple had lived happily for a while and had had two children. Eventually, the woman had realized that her husband was not aging like everyone else. She left him, fearing that he and his family were "witches, or worse." One of the few concrete clues about the unusual situation that the man in the yellow suit had been given by his grandmother was the information that the mother in the family had had a music box, which had played a distinctive, haunting melody. The man had recognized the tune coming from the wood by the Fosters' house, the day before Winnie had been taken away by the Tucks.

Having gained ownership of the wood from the Fosters in exchange for the safe return of their daughter, the man in the yellow suit now intends to market the magic water and make a lot of money. Tuck is horrified by this news and tries to tell the man why his plan would bring about disaster, but his entreaties fall upon deaf ears. The man in the yellow suit grabs Winnie and is dragging her out of the house, when Mae Tuck accosts him at the door, holding Tuck's shotgun by the barrel like a club. Swinging the shotgun, she hits the man in the head, and he drops to the ground, senseless, just as the constable from Treegap comes upon the scene.

Winnie has changed during the short time she has been away. Although she still loves her family dearly, she now has new, conflicting ties, "tugging and insistent," binding her to her friends the Tucks. When she returns home, Winnie insists that she had gone with the Tucks of her own accord, so charges of kidnapping are not brought against them. The man in the yellow suit dies, however, and Mae is placed in custody, to be executed for murder. Winnie knows that if Mae is sent to the gallows, the world will witness the fact that she cannot die.

The next morning, Winnie is sitting outside in the yard, when Jesse comes by. He tells her that Miles has a plan to free their mother from jail, but that since the constable checks on Mae constantly, they will have little time to get away. There are no other options, however, so Jesse has come to say goodbye. He gives Winnie a small bottle of water from the magic spring. He urges her to drink it when she is seventeen and come to them.

Winnie suddenly has an idea, and tells Jesse that when Miles removes the window of the jail to allow Mae to escape, she, Winnie, will climb in to take her place. In the darkness, the constable will not notice the difference, giving the Tucks more time to get away. That night, a thunderstorm is brewing, and its noise conceals the sound of Miles dismantling the window. Mae squeezes out, and Miles lifts Winnie in as planned. When the constable comes to check on his prisoner, he does not realize that it is Winnie huddled under the blankets until morning.

The Tucks get away safely, and Winnie faces censure for what she has done. Her family is distraught, but somehow, they eventually come to understand that she has helped the Tucks "because—in spite of everything, she love[s] them." One day, in her yard, Winnie rescues a toad from a marauding dog, and impulsively pours over it the precious water from the bottle Jesse has given her, thinking that if she later decides to meet him as he has asked, she can get water directly from the spring. Setting the toad free, she tells it, "There! You're safe. Forever."

Almost seventy years have gone by when Mae and Tuck pass through Treegap again. It is 1950, and the wood is gone; the Tucks learn from a local resident that about three years previously, the big tree at its center had been split by lightning and had caught fire. The whole area has been bulldozed out since then, and no one knows what happened to the fresh water spring that Tuck remembers.

Tuck stops at the cemetery before he and Mae move on. There, he discovers the grave of Winifred Foster Jackson, who had been a "Dear Wife [and] Mother," and had passed away in 1948 at the age of seventy-eight. In the peaceful silence, Tuck salutes Winnie briefly, saying, "Good girl," then quickly walks away. When he tells Mae the news, she comments wistfully, "Poor Jesse," even though they had long known that Winnie would not be coming to join them. As they head down the road, the old couple sees a toad sitting placidly on the asphalt as a truck whizzes by. Tuck stops the wagon and picks up the bold creature, and, placing it carefully on the side of the road, mutters, "Durn fool thing must think it's going to live forever." Tuck and Mae then leave Treegap behind, accompanied by the "tinkling little melody" of Mae's music box.