The alien invasion is over, and, uh, we kind of lost. Oops. The story opens with an alien parasite being implanted into a human body. They call the alien Wanderer, and Wanderer's job is to access the memories of her host body, Melanie, and tell the Seeker where to find more humans.
The thing is, Wanderer starts liking Melanie. More than that, she likes the way Melanie's memories of her hunky boyfriend, Jared, make her feel. Wanderer wants to find Jared and Melanie's brother Jamie herself. To make sure they're safe and keep them that way.
Deciding to betray her species, Wanderer sets off into the desert and uses Melanie's memories to find Jared and Jamie's hideout. Unfortunately, she's not very good at resource management. She guzzles through their water supply way too fast and passes out from exhaustion in the desert. Don't try this at home, kids.
Melanie's Uncle Jeb finds Wanderer/Melanie in the desert and gives them a drink of life-giving water. They revive, only to find themselves confronted with a dozen or so angry, alien-hating humans. With guns. Dehydrating in the desert is starting to look good after all...
She finds out that Jamie and Jared are still alive, but Jared is so not happy to see her. He greets her with a backhand to the face. Yikes. Well, it's not like Emily Post has an etiquette tip on what to do when your girlfriend has been taken over by an alien parasite. For all we know, this might be the preferred method of action. At least he didn't shoot her.
Jeb doesn't want Wanderer/Melanie dead, though. And since he's in charge, what he says goes. He puts Wanderer/Melanie into a tiny little prison cave, where they keep her fed and watered like a scraggly houseplant until they decide what to do with her.
Over the next few weeks, Wanderer slowly acclimates to cave life, and the humans kind of get used to her, too. Jamie doesn't think she's half bad, and becomes like a brother to her. They start calling her by the less creepy name of "Wanda." Wanda tells them stories of her lives on other planets. One of the cave dwellers, Ian, might even be falling in love with her. Life is fairly calm for a bunch of alien-resisting rebels.
Unfortunately, Walter, one of the cave dwellers, has cancer and is dying. On his deathbed, he mistakes Wanda for his deceased wife Gladys. She goes along with it, offering him comfort and compassion until he dies. This compassionate act endears her to Doc, the cave's flamenco dancer. Kidding, he's a doctor.
Not all the humans like her, though. Jared loves Melanie, but still kind of hates Wanda. Ian's brother Kyle hates her enough to try to drown her in the bathing cave. He doesn't succeed, but Wanda refuses to let the others punish him. She believes that most humans are ruled by their anger, and Kyle isn't any different.
One day, Jared and Jamie return from a supply run. It was Jamie's first, and he fell on his knife like a doofus. His injury becomes infected and life-threatening (worse part about the alien invasion/zombie apocalypse: no more penicillin), so Wanda convinces Jared to take her to a healing facility in Tucson. There, Wanda deceives the souls and steals their awesome drugs. (The legal, healing kind.) Her heroism saves Jamie, and everyone pretty much loves her after that. Except Kyle. He's still a jerk.
Seeing the benefits to having Wanda on their side, Jared takes her on a supply run. She can shop in alien-run stores without raising suspicion. They return from a supply run and discover that the Seeker killed a cave dweller and has been taken hostage.
Wanda doesn't want the Seeker to be killed. She believes she can save both the Seeker and the body she resides in. Wanda performs a surgical procedure safely removing the Seeker. It's much less gory than Doc's previous efforts, which always resulted in the death of alien and host. They put the Seeker in a cryotank and blast the Seeker into space. Don't let the moon hit ya on the way out!
They experiment on a few more body-snatched humans, sometimes saving the human, sometimes not. When Doc has practiced enough on the red shirts, Wanda decides it's time to give Melanie her body back. While Wanda is in her cryotank, Jared, Jamie, and Melanie go find a new body for her. They stick her in, and she's reborn!
Ian tells her he still loves her. Jared loves her, too... loves her to do his shopping. They take her on more supply runs, where they meet another group of humans. These humans also have an alien living amongst them, making us wonder how this alien/human co-existence will work out in the long term and setting us up for a sequel.