a species of parasitic aliens called "souls" have invaded earth, deeming the humans too violent to deserve the planet. when a soul is implanted into a host body, the consciousness of the original owner is erased, leaving their memories and knowledge. wanderer, a soul, is placed into the body of melanie stryder. however, melanie's consciousness is still alive and begins to communicate with wanderer mentally. wanderer's assigned "seeker" suggests that she should find a way to remove melanie through surgery, but wanderer makes several attempts to deny her seeker's wishes. as wanderer starts to uncover some of melanie's memories of her younger brother jamie stryder and her boyfriend jared howe, melanie gets her to follow a series of landmarks throughout the arizona desert to find her uncle jeb, hoping that jared and jamie are with him. by doing so, she would be denying the seeker's ability to get melanie out of her mind.