The answer to your question can be found in the prologue of this book. The author tells us plainly that three different events took place, and that, at face value, there seemed to be no connection between them.

The first occurrence is Mae Tuck heading out on a trip to visit her two sons in the woods where they live, just outside of the village of Treegap. This is a ritual that takes place every ten years, which would not seem nearly often enough for a mother to see her sons without the context of knowing that this family is immortal.

The second event that we are told about involves Winnie Foster deciding that it is time to take the idea of running away seriously. Winnie is from a wealthy family, and they are the owners of Treegap Wood.

The final event which appears not to be connected involves a stranger arriving at the gate to the Fosters' property. We are told that "he was looking for someone, but he didn't say who."

The most important thing you need to know about these events is that while, at face value, they do not appear to be connected, they ultimately do prove to be connected. These three incidences are going to come together in the course of the story to forever change the lives of our protagonists.

The prologue to Tuck Everlasting lists three seemingly unrelated things that happen one August day. At dawn, Mae Tuck sets out for the woods at the edge of the village of Treegap, where she intends to meet her sons, Miles and Jesse, as she does once every ten years. At noon, Winnie Foster begins to think about running away. And at dusk, a stranger appears at the Fosters' door looking for someone.

The three events turn out to be very much interrelated, and they produce a conflict disastrous enough to cause "this weary old earth" to "trembl[e] on its axis like a beetle on a pin." The plot of the book unfolds as these three events come together.

The next day, Winnie acts on her plan to run away by heading into her family's wood, which is where Mae plans to meet Miles and Jesse. Winnie meets Jesse first and sees him drinking the miraculous water. In order to keep their secret, Mae, Jesse, and Miles kidnap Winnie.

The stranger who had appeared at the Fosters' gate the previous night was looking for the Tucks. Because Mae carried her music box into the wood and played it as he was standing at the Fosters' gate, the man suspects that one of the Tucks is in the wood, so he observes as the Tucks set out on the road with Winnie toward their home. He follows them and eavesdrops on the story when the Tucks explain the secret of the water to Winnie.

Now the three events have intersected, and with the stranger's nefarious intentions to sell the water to the world, a cataclysm looms...