It never entered my mind, while reading this book, how much like a classic fairy tale it is. The gift of immortality, the villain, even the toad that may be more than he seems - is he an enchanted prince? (Spoiler alert: no.)
Winnie accidentally discovers a family that has (accidentally) become immortal; they kidnap her to prevent her from revealing their secret; she goes through a whole range of emotions from fear to disgust at their less-than-perfect home life to, eventually, love. The family explain the different positive and negative aspects of living forever, and we see that immortality isn't necessarily a good thing. Winnie has a chance to become a better person (and does), and a chance to become immortal herself (I won't tell you how that works out), and a common garden toad becomes immortal.