Earth has been taken over by aliens who call themselves souls. They take over human bodies (the organism that effects the environment the most) and live out their life in that guise.
Wanderer, one of the most experienced souls, has requested an adult body, almost unheard of in her day. Adults are the more difficult to control, as they have been influenced by outside forces for longer than children. As one of the strongest souls in their society, Wanderer is expected to use her new body to find the hidden pockets of resistance and help make Earth more peaceful.
What no one expects is that Melanie is very much alive in her own body. Shunted off to the side by Wanderer's soul, Melanie clutches on to her very being--unwilling or unable to let go completely. As she and Wanderer spend more time together, however, Wanderer begins to understand the complexities behind human existence. Forced to see the evil nature of her own species through the eyes of the very organisms she has been trained to discard, Wanderer cannot resist Melanie's pull to find her family and help them survive.
Even though this book is primarily science fiction, it has appeal across all genres as it examines what it means to be human. There are definitely some slower parts to the novel--Meyer has written three teen novels--this is her first adult attempt. The examination of what makes us human, and what can still be admired about our own species is a nice change from everyday life.