In the near future, the earth is completely at peace. There is no hunger, no war, and everyone lives together in kindness and with respect for each other and the planet. This is because the earth has been invaded by an alien species, that has taken over the control of almost all the human bodies living on earth, except for a few renegade humans hiding from capture. A group of aliens called the Seekers are tasked with hunting down the remaining humans so they too can be transformed into host bodies for the aliens to inhabit. A young human woman, Melanie Stryde (played by Saoirse Ronan) has avoided capture along with her younger brother. They sneak into an abandoned hotel looking for food and are discovered by the Seekers lead by Lacey (played by Diane Kruger). Melanie tells her younger brother to hide under a desk while she distracts the Seekers away from him and promises to return to him. The Seekers corner her. In order to escape she jumps through the glass of a window on a high floor and falls to the pavement getting badly injured but she does not die.
The Seekers have advanced medical equipment which causes instant healing. They apply this to Melanie. They then cut her neck only slightly and gently insert an alien life form into the opening which glistens under the skin as it crawls up the back of her neck to enter the brain and take over the body. This alien being calls itself the "Wanderer." It is more than a thousand human years old, having lived many lifetimes as an invader in other species' bodies. Melanie's consciousness still exists inside her body. Both Melanie and the Wanderer co-exist within the same body fighting for control of it.
Melanie tricks the Wanderer into running away and forces her to crash the car in the desert leaving no choice but to walk to the hideout of the humans where Melanie's boyfriend and her younger brother live. The humans find her in the desert, nearly dead from lack of water. They take her inside the compound which is a hidden cavern where many humans have taken refuge. At first, the humans want to kill her thinking she is only an alien, but she convinces them that she is also part human and that both the alien life force and the human one are living together in the same body.
Melanie's brother gets hurt and he is close to dying, The only way to save him is if she goes to the alien healers and steals the technology needed, which she does proving to the humans that she is not their enemy but their friend. The Seekers are still looking for her and the seeker Stacey does eventually find her. They fight and Stacey is shot. The alien is removed from Stacey's body, so she becomes human again. Now that they know it is possible to remove the aliens from the human bodies and restore them back to humans, they begin to restore humanity back on earth.
Best part of story, including ending:
I really like the way the aliens can transport across the galaxy and that they are interesting luminescent delicate creatures that are not really malevolent just misguided.
Best scene in story:
The scene where they use the advanced medical technology that causes Melanie's younger brother to be immediately restored to health and his wound to fully heal in a matter of moments, is an amazing special effect.
Opinion about the main character:
Melanie is tenacious and not giving up the fight for control over her body very easily. This struggle and the reasons why she is doing it, for her boyfriend and for her younger brother are what eventually convinces the alien of her true nature, which the alien then comes to respect.