We think we understand the laws of physics. We think a lot of things. It was perhaps inevitable that some of them would turn out to be wrong. When the great incursion occurred, no one was prepared. Of all the things physicists had predicted, "the fabric of reality might rip open and giant monsters could come pouring through" had not made the list. But somehow, that was precisely what happened. For sisters Susan and Katharine Black, the day of the incursion was the day they lost everything. Their home, their parents, their sense of normalcy . . . and each other, because when the rift opened, each sister was stranded in a separate form of reality. For Susan, it was science and study and the struggle to solve the mystery of the altered physics inside the zones transformed by the incursion. For Katharine, it was monsters and mayhem and the fight to stay alive in a world unlike the world of her birth. The world has changed. The laws of physics have changed. The girls have changed. And the one universal truth of all states of changed matter is that nothing can be completely restored to what it was originally, no matter how much you might wish it could be. Nothing goes back.