Amazon has a succinct summary, so why reinvent the wheel?
"In 1970s London, eight-year-old Peggy Hillcoat lives somewhat contentedly with her survivalist father and her concert pianist mother. When her mother goes on tour, her father abruptly kidnaps Peggy, taking her to a German forest. He claims that the world has ended and that her mother, along with every other human on Earth, has died. She resigns herself to a life in the cold, remote woods with her mentally unstable father, little food, and no medical care, not resurfacing until 1985."
The story is told by 17-year-old Peggy, going between present day as she reacclimates to life in 1985 and flashbacks of her life in the woods with her father. The piecing together of the story definitely holds the reader in suspense, always wondering what Peggy will endure next? However, it's the ending that makes you double take, doubting yourself and reexamining what you thought was and what could actually be.
And that's all you're getting! Go read!