Our world has been invaded by an unseen enemy. Humans become hosts for these invaders, their minds taken over while their bodies remain intact. Most of humanity has succumbed. When Melanie (Saoirse Ronan), one of the few remaining "wild" humans, is captured, she is certain it is her end. Wanderer, the invading "soul" who has been given Melanie's body, was warned about the challenges of living inside a human: the overwhelming emotions, the glut of senses, the too-vivid memories.
But there was one difficulty Wanderer didn't expect: the former tenant of her body refusing to relinquish possession of her mind.
When outside forces make Wanderer and Melanie unwilling allies, they set off on a dangerous and uncertain search for the man they both love.