Rachel and Alice are an extremely rare form of identical twins—so identical that even their aunt and uncle, whom they've lived with since their parents passed away when the girls were nine, can't tell them apart. The sisters are connected by an intense bond that goes way beyond their surfaces and borders on the supernatural: when one experiences pain, the other exhibits the exact same signs of stress, too. So when troubled Alice disappears mysteriously one night, good-girl Rachel knows something is wrong-especially when Rachel starts experiencing serious physical traumas, even though nobody has touched her.
What Rachel can't tell anyone is that she and Alice sometimes switched places, reveling in the possibility of being the "good one" or the "bad one" for a day. And that Rachel . . . is really Alice, continuing to masquerade as her twin. So then, what happened to her sister? Could whoever abducted her sister really have meant to take her, instead? And can she find the real Rachel before it's too late for both of them?