First things first: the narrator explains the setting. We're in the woods near Treegap, where there's an important spring, owned by the Foster family. Got it? Okay, here we go.
Mae Tuck is getting ready to go meet her sons. But before she leaves, Mae and her husband Tuck have a conversation about wanting to change the things they can't. Sounds normal enough, right? Oh, but then the narrator hints that these two are immortal. Well, then.
Winnie Foster (of the Foster family—remember them?) is talking to a toad when a strange man in a yellow suit appears and asks her questions. Winnie's grandma joins them and they all hear some strange music. (Don't worry, we're confused, too.)
The next day, Winnie runs away, and she meets Jesse Tuck, who's drinking from a spring. Dude won't let her have any of the water, though. Soon enough, Jesse's mother and brother, Mae and Miles, appear. The three Tucks grab Winnie and start running—but not before Yellow Suit Guy can see them. When they finally get a moment to stop, the Tucks tell Winnie their story, and Winnie promises to keep her mouth shut about the whole thing.
Here's the story: Eighty-seven years ago, the Tucks came to Treegap and drank from this spring. Time went by, Miles started a family, and things started to get weird: the Tucks weren't aging and they couldn't die. Miles's family left him, and the Tucks had to move away. When they finally figured out that drinking from the spring was what did this to them, they turned to a life of secrecy.
After they finish their no-big-deal story, the family takes Winnie to meet their patriarch, Tuck. (Of course, none of them notices that Yellow Suit Guy totally overheard them. We have a feeling that won't end well.) During dinner, Winnie starts to freak out. That's when Tuck takes her out on the pond for a serious talk about life and death. But Miles quickly interrupts them because their horse has been stolen.
Meanwhile, Yellow Suit Guy, who stole the horse (surprise surprise), is headed right back to the Fosters' house. Just like that, he makes the Fosters trade him their land for Winnie's safety. So back at Chez Tuck, Yellow Suit Guy barges in and reveals his evil plan: he owns the spring now and he's going to sell the water. What's worse—he's going to make Winnie drink it. Mae's answer? She bashes his head in with a shotgun. Nice.
Just then, the constable rolls up. He takes Mae to jail, and Winnie goes home, unable to explain to her family why she went with the Tucks. As it turns out, Yellow Suit Guy died, which means Mae is a murderer. Winnie and the Tucks are able to break Mae out of jail, but it means they have to say a very sad goodbye.
A few weeks later, Winnie makes the big decision not to drink the immortality water that Jesse had given her. Instead, she gives it to a little toad. She figures she can always return to the spring at another time—if she wants to.
Flash forward to 1950. Mae and Tuck come into Treegap only to learn that the forest and spring are gone. When Tuck visits the local cemetery, he sees Winnie's grave. Looks like she passed up the immortality water forever, after all.