It is the not-too-distant future; parasitic aliens have invaded the Earth. These aliens, called "Souls," look like the dust residue that accumulates in the bristles of a broom after an hour of sweeping, but colored a luminous white and silver: genus body-snatcher, phylum eerily benevolent. Souls do not lie, cheat, or steal. They do not kill each other. They share all goods in common, and none seek for private gain.
Souls would be completely peaceful if it weren't for the last pesky humans trying to remain alive—like Melanie (Saoirse Ronan), her little brother Jamie (Chandler Canterbury), and her boyfriend Jared (Max Irons), who are heading for the safety of an uncle's desert cabin.
The Seekers are the Souls' police force, and when Melanie goes scavenging, one captures her. The Seeker (Diane Kruger) puts a Soul named Wanda (also played by Saoirse Ronan) into Melanie's body, who tries to comb Melanie's memories for clues to the location of other survivors. Melanie survives the possession and remains as a voice inside Melanie/Wanda's body, using the strength of her will to resist. Wanda's control is tenuous—Melanie can struggle against her mental bonds and regain physical control for brief moments.
As Wanda and Melanie cohabit the same body, they start to learn about each other—and when the Seeker threatens to remove Wanda from Melanie's body unless she starts discovering new information soon, Wanda decides to flee her kind and go join Melanie's family in the desert. She discovers that the last bastion of humanity lives in a complex network of underground caves. Melanie's uncle Jeb (William Hurt) leads the humans, and he protects Wanderer from skeptical ...1