Earth has been invaded by an alien species known as Souls, who take over human bodies and erase the hosts' consciousness to establish a peaceful and compliant society. Melanie Stryder, one of the remaining human rebels, is captured and implanted with a Soul named Wanderer, meant to navigate her memories to discover the location of the remaining human resistance. However, Melanie's consciousness refuses to relinquish control of her mind, and she and Wanderer become reluctant allies. As Wanderer accesses Melanie's memories, she is overwhelmed by the human's past experiences and emotions, particularly her relationships with her younger brother, Jamie, and her boyfriend, Jared.

Compelled by Melanie's strong feelings and her own developing empathy for humans, Wanderer sets out to find Melanie's loved ones, leading to an unprecedented bonding between host and alien. This journey brings them to a hidden rebel enclave, where the humans are initially hostile, distrusting Wanderer despite Melanie's internal pleas to listen to her. As Wanderer struggles to prove her intentions and adapt to the community, she becomes drawn to another rebel, Ian, further complicating the situation. Melanie and Wanderer's coexistence challenges the perceptions of both species, questioning the invaders' concepts of unity and the remaining humans' understanding of what it means to be alive.