"Tuck Everlasting" is about a family called the Tucks who drank from a magic spring that grants complete immortality. It is about the interactions between the family and a young girl named Winnie Foster, and their attempts to keep her from drinking from the spring.
The Tucks want to keep the spring secret because they have discovered, through years of experience, that immortality is more curse than blessing. They cannot stay in one place for too long or people get suspicious, and the eldest son Miles already had a family leave him out of fear.
One day, Winnie Foster is traveling through the forest, comes across the spring and nearly drinks from it until the Tucks stop her and take her to explain why immortality is unnatural. She eventually understands and refuses to drink the water. This is shown by the Tucks returning to their town a hundred years later and seeing Winnie's tombstone, also noticing that the tree and spring are gone.
The book makes use of the themes of life, death and the natural order to explain why death is not an inherently bad thing. Without death, there would be no room for new life or growth, and everything would become stagnant.