The book starts with Melanie (the human host) lying unconscious in an operating room. You find out that she tried to kill herself by jumping into an empty elevator shaft but you don't know why. A Soul (the alien "parasite") named Wanderer is implanted into her body in order to access Melanie's memories and to find any information she might have about human survivors.
When Wanderer wakes up (in Melanie's body) the first thought she has is Melanie's last memory. We discover that Melanie jumped into the elevator shaft because she was about to by captured by Seekers (Souls who sought out "wild" humans) and would rather be dead then become a host. But the elevator shaft wasn't deep enough - Melanie didn't die which is why the Healers (Souls in the medical profession) were able to save her body and implant Wanderer.
When Wanderer tries to search Melanie's memories for more information she finds that the memories are blocked. All she can discover is that Melanie traveled from a desert place to Chicago to try to save her cousin Shannon who might still be a wild human. Wanderer is confused as to why she can't access all of Melanie's memories until she discovers that Melanie is still alive - Melanie's presence is still living in the body guarding certain memories from Wanderer.
Over the next few chapters we discover that Wanderer was so named because she has lived on about eight different planets (the norm is about three planets) but never found a planet to call home. She is working as a lecturer at the local college teaching her students about the different worlds - sort of like a history teacher who actually lived through history. We also meet the Seeker assigned to Wanderer. Wanderer is supposed to give as much information to the Seeker as she can but since Melanie is blocking the memories Wanderer isn't being very helpful. The Seeker becomes suspicious and for good reason...
Melanie has become bold enough to speak to Wanderer. Wanderer is concerned because humans are supposed to fade away once Souls are implanted into their bodies. But Melanie is sticking around, protecting her secrets and making it very clear that she does not like Wanderer. However, Melanie sometime becomes bored and let's her mind wander and so Wanderer eventually discovers that Melanie was hiding out with her boyfriend, Jared, and her little brother, Jamie. Eventually, Melanie's feelings of love and protection spill over to Wanderer until Wanderer can't tell which are her feelings and which are Melanie's.
Which is why Wanderer decides to run away from the Seeker to try to find Jared and Jamie. Wanderer was driving to Phoenix (or somewhere around there) in order to "skip" bodies - basically, she was going to give up living in Melanie's body and move to another, more compliant host. But along the way Melanie convinces her to search for her Uncle Jeb who is sort of crazy and living in his bomb shelter somewhere in the desert. This is the safest place for Jared and Jamie to be and since Melanie is excited about finding them Wanderer is too.
They travel by car as far as they are able and then they start to walk. Melanie's body is used to harsh conditions but Wanderer is not and so the two of them have a hard time coping with each other as they travel. Eventually they run out of water and both of them think they are going to die... but Jeb finds them just in the nick of time. He has a group of survivors with him (among them two brother named Kyle and Ian) who insist on simply killing the body since it's not really Melanie anymore anyway. Jeb insists on bringing the body back to the shelter (which is a huge underground cave network) to let "him" decide what to do with the body.
Turns out the "him" that Jeb was talking about was Jared who did manage to bring Jamie safely to the caves. Now it gets complicated. Melanie's body is still alive but the humans think that Wanderer is the only presence living in that body. Melanie insists that Wanderer tell her family the truth, that she is still alive, but Wanderer is scared about what the humans will do to her if they find out so she keeps her mouth shut. Jared can't bring himself to kill Melanie's body but he can't look at her while she's a host so he keeps Wanderer in one of the storage closets and feeds her bread and water until he can decide what to do with her.
However, there are a few people in the community who would rather see Wanderer dead. The two brothers from before, Kyle and Ian, ambush Jared and almost kill Wanderer until Jeb comes in and lays down the law - he threatens to shoot the brothers unless they behave. As Jeb puts it, "My house, my rules. If you don't like it, you can leave." The brothers aren't happy but they don't want to leave so they grudgingly oblige. Eventually Jared and Kyle and a few others go out on a raid to get more supplies leaving Wanderer in the care of Jeb.
Jeb takes pity on Wanderer and moves her to a more comfortable living arrangement - which turns out to be Jared and Jamie's living quarters. Wanderer isn't comfortable with this but Melanie is ecstatic. Especially when Jeb allows Jamie to come visit Wanderer. It's at this point (when Wanderer starts interacting with the community more) that Jeb starts to suspect something is different. He begins to call her Wanda in an effort to integrate her into the community. He also makes her do her share of chores and eat in the cafeteria in order to get everyone comfortable living together. And surprisingly Ian ends up being one of his greatest supporters. Wanda is obviously quit skeptical of Ian's intentions but as they get to know each other Wanda begins to trust him more and more. Melanie does not like where this is going.
On a side note, I should also point out that over the course of the book you discover a lot about the history of the Souls. They started out on a planet they call The Origin but when their species was attacked by Vultures they implanted themselves into Spiders in order to defend themselves. (The names of the alien creatures are rough translations into our language. Throughout the book Wanda complains that she knew the name of such-and-such an object when she was a Spider, or that there is a useful phrase which only makes sense in Bear.) The Spiders, instead of putting up a fuss, welcomed the Souls and the peace that they brought since Souls are naturally peaceful creatures. Encouraged, the Souls discovered new worlds, implanting themselves into the dominant life forms and creating peaceful societies. Some of the worlds that Wanda has lived on include the Flower world, the Singing world, the See Weed world (yes, See Weed), the Bat world, and the Bear world. There are others which include the Fire world and the Dolphin world which Wanda has never been to. In this new community she becomes a sort of teacher, answering questions about the other worlds for people who are curious.
Back to the story: Jeb figures out that Melanie is still living inside Wanda's head. And then, of course, Jamie finds out. When Jared and and the rest of the raiding party get back Jared finds out too. In order to test the hypothesis, Jared kisses Wanda and Melanie becomes extremely jealous. She takes control of her body for long enough to punch Jared which is enough to convince Jared that Melanie is still alive. Jared agrees that they should give Wanda her own living arrangements which Kyle isn't too please about. He insists that Wanda should die anyway. Jared is upset because this would kill Melanie too; Ian is upset because he's protective of Wanda. Jamie doesn't want his sister or his friend to die. Jeb says that Wanda is a guest in his house and will be treated as such. Kyle goes off to sulk.
Around this time one of the members of the community starts to die from cancer. He's been sick for a while but since there are no medical supplies there was nothing the resident doctor could do for the man. When he starts to feel pain the doctor makes him drink whiskey. When the man becomes delusional he asks for Wanda. Wanda helps him the best she can (which entails holding his hand and helping him drink his whiskey) but is confused as to why he would ask for her in the first place. The doctor explains that the man is confused - his wife was captured by Seekers and he's confusing his wife's story with Wanda's. Around this time a small raid party (which includes Jared and Jamie) goes out for a small amount of necessary supplies.
Wanda takes a break from comforting the dying man early one morning to take a bath in the hot springs before everyone else wakes up. She's in the "wash room" by herself for a while until Kyle shows up and takes advantage of the opportunity to try to kill her again. He beats her up pretty bad and tries to drown her but the floor starts to give away beneath them right where the water from the springs disappears into an underground river. Wanda is light enough to crawl away but she feels guilty about leaving Kyle who is knocked unconscious. She crawls back and tries to drag him to safety but the floor can't hold both of them. She braces herself against the rocks and calls for help. Ian shows up and first saves Wanda and then goes back for his brother.
When he asks Wanda what happened Wanda lies and says that Kyle didn't try to kill her, that she tripped. Ian doesn't believe her. In fact he's furious at his brother and threatens to give him the same bruises and scars that Kyle inflicted on Wanda. Wanda steps in to convince Ian not to fight with his brother. Kyle doesn't understand her kindness. Ian is upset that she won't let him protect her. It's no secret anymore that Ian has a thing for Wanda. He even kisses her forehead when he drops her back off at the infirmary.
When the raiding party comes back, they have found enough morphine to make the dying man comfortable in his last moments. Wanda stays with him until he's gone. Then they hurry her out of the infirmary to "drop off" a few more supplies that she's not allowed to see. But Melanie convinces Wanda to find out about the supplies anyway so they sneak back to the infirmary where they discover that the extra load of supplies is actually several host bodies and that the doctor is trying to take the alien soul out of the human body to try to save the human. But every time he preforms the operation both soul and body die. Very graphically. When Wanda walks into the operating room she starts screaming and going into hysterics. Jared wonders what's wrong since the dead bodies are covered but Ian points out that the dead human bodies are covered, not the dead souls.
He tries to comfort Wanda but she runs away and hides in the caves for a couple days. Melanie tries to comfort her but Wanda pushes her away. Over the course of about three days Ian comes to look for her leaving her food and water which she doesn't eat. Jared come to look for her but Wanda won't talk to him. Eventually Jeb come to see her and is finally able to get some response out of her. She cries for a while, she finally eats something, and everything seems to be going OK until Jeb tells her that Jamie had been hurt while out on the raid.
Wanda is finally able to focus on something other then the dead souls. She hurries to Jamie's room where she finds him fighting a fever from an infected wound. She is surprised to find that only she is concerned - Melanie is no where to be found. In a panic she rushes from the room, Ian following close behind to make sure everything is OK. In order to get Melanie jealous (an emotion that caused a stir with Melanie last time) Wanda kisses Ian. Nothing happens. Wanda goes to find Jared and explains the situation and asks for his help. Jared kisses (and I mean passionately kisses) Wanda until Melanie realizes what is going on and pushes him away. Ian is obviously jealous.
When Wanda asks Melanie where she was Melanie can't remember. She just sort of disappeared for a few days. It's at this point that Wanda realizes that she and Melanie can't share the same body forever - eventually one of them is going to have to fade away. Wanda is worried that Melanie is going to be the one to go. But first things first, they have to work together to save Jamie.
Since Wanda is a Soul living in a human body she has the outward markings of a soul - reflective eyes, sort of like a cat - and therefore would not be noticed if she walked around in civilization. She suggests faking Jamie's injuries on herself, receiving medical attention from a Healer, and stealing the necessary medicines to bring back to Jamie. No one else approves of the plan except Jared who sneaks her out of the caves later that night and drives her to the nearest city. In order to successfully pull off the plan Wanda has to make it look like she had a camping accident which means not only stabbing herself but also re-opening a few scars she received earlier when Kyle tried to kill her. Jared seems hugely impressed that she would do this to save a human. Wanda reasons that since Jamie needs her help why wouldn't she sacrifice herself? Wanda has her own set of feelings for Jamie apart from Melanie.
When Wanda is in the hospital she's amazed at the drugs that they use to help heal her. Each drug is labeled with a generic name - No Pain for pain relievers, Cool for fever reducers, Cleanse for antiseptic, Heal in place of stitches, Smooth for scar reduction, etc. They use all of these on her to cure her "camping accident" and in five minutes all traces of her injuries are gone including the scars from before. She steals a few bottles of each and brings them with her back to the caves. The doctor is amazed at how fast the drugs work because Jamie is up and about in no time at all - and I mean within 5 minutes. In fact, they give him the Cool and No Pain right away which causes Jamie to come around before they even have time to administer the Clean and Heal to the injury on his leg.
Now that the rest of the community knows that Wanda can be trusted they send her out to do all the raids. She travels with Jared, Ian, and Kyle and simply shops for what they need. Since the Souls don't have a form of currency they don't have to worry about money. Wanda simply walks into a grocery store, loads up on non-perishables, and walks out again. She's a bit nervous about "stealing" but Melanie convinces her that she's actually doing something good since she's helping people even though she's not helping souls.
On one of their trips the four of them notice a family (parents and a toddler) playing in a playground across the street. Wanda says that's the closest she's come to seeing hope for the survival of a species on all the planets she's been on. Jared asks her what she means and she tells him that the parents are Souls but the baby is a human baby. No Soul would run around so carelessly or happily she explains. The parents weren't able to give that up.
When they get back to the cave they are told that Wanda has a visitor. It turns out that the Seeker that she ran away from in the beginning has been trying to track her down this entire time. Since operating on a Soul/human obviously is upsetting Wanda they wait until she gets back so she can tell them what else can be done. Wanda is scared of the Seeker at first thinking that the Seeker is going to take her away from her family but finally Wanda decides to make the ultimate sacrifice....
She offers to tell the doctor how to take a Soul out of a human body without killing anyone on one condition - that the doctor take her out of Melanie's body. Wanda has realized that there is no other planet she would be happy on so she tells the doctor that when he takes her out of Melanie's body he needs to simply let her die. Weighing the options the doctor decides that this is the best offer that he's going to get even though he'll be sad to see Wanda go. He agrees. She makes him swear.
She and Jared (and I think Ian) go on another raid to get a hibernation tank for the Soul living in the Seeker's body. They steal one and head back to the caves without a hitch. They knock the Seeker out and carry her to the infirmary where Wanda explains the simple procedure of extracting a Soul. Every Soul has a sensitive spot on their underbelly that, if pressed, causes them to release their hold on their host. Wanda does it the first time showing the doctor how to hold the Soul and place it in the hibernation tank. Once that is done the Clean, Heal, and No Scar are applied to the small incision on the back of the person's neck.
Surprisingly, the person who was once the Seeker wakes up. She begins complaining loudly that she was trapped inside her own head for several years much the same way Melanie is trapped. This gives the humans hope that they can save other people. Wanda makes the doctor swear that the Souls he extracts he will keep safely hibernated until they can be placed on a shipment to another planet. He agrees. At this point Kyle disappears and Ian correctly guesses that he's gone to find his wife who was captured by a Seeker several years ago.
When Kyle finally returns he has indeed found his wife. Wanda takes the Soul to the infirmary where she tries to explain to her what is going to happen. Wanda asks if there is a specific planet she would prefer to be sent to. The Soul asks if she can stay and Wanda says no, she has to leave. The Soul asks why Wanda is staying. Wanda explains that she is planning on leaving too... just in time for Ian to hear. He gets really upset, dragging Wanda out of the infirmary and back to his room where they can talk in private. He confesses that he loves her and she confesses that she loves him... but she loves Melanie too and she can't keep her trapped forever, not when Jared and Jamie need Melanie. Ian refuses to let her go and even calls a council to vote on it. Everyone (even Melanie, who can't really talk but tries to anyway) agrees that Wanda should stay since she is more of a help to the community.
Wanda lets them have their say but she knows that she made the doctor swear to keep his end of the bargain. Melanie tries to talk her out of it but even Melanie can't convince Wanda that her staying would be a good thing. Wanda stays around long enough to supervise the extraction of the Soul from Kyle's wife. They try to wake up Kyle's wife but she's taking a while to come around. The finally give up for the night and decide to get some rest. Wanda sleeps in Ian's room since he won't let her out of his sight in fear that she'll try to leave him.
She waits until Ian is asleep and she sneaks out of the room. Melanie says that she'll say goodbye for her. She and Melanie have a heart-to-heart goodbye of their own - they've grown to love each other in the time they've shared the same body. Just before they get to the infirmary Wanda runs into Jared. Jared knows exactly what she's doing but still says that he'll miss her. They've become friends in spite of themselves.
Wanda shows up in the operating room and the doctor is waiting for her. He asks her one more time is she is sure she wants to do this and she says yes. He gives her No Pain and something else to knock her out. She lets herself float away into nothingness....
Luckily for us (and for Wanda) there is an epilogue. Just as in the beginning of the book Wanda is waking up inside a human. Wanda's first thought is the body's last memory. She remembers her old face (the face of Melanie) coming to her claiming that she is lost and asking for directions. Melanie leads her into an alleyway where she sees Jamie and Jared waiting for her. They give her something to knock her out and carry her back to the caves.
As Wanda is coming around she realized that the memory doesn't belong to her. It belongs to the previous host which is why it's a little fuzzy. When she finally wakes up she realizes that Melanie, Jared, Jamie, Ian, and the doctor are all there waiting for her to wake up. They explain what happened:
After they said goodbye, Jared realized that he couldn't let Wanda sacrifice herself. He ran into the operating room and held the doctor at knife point until he promised to go against Wanda's wishes and put her in hibernation instead of letting her die. Once she was safely in hibernation they woke up Melanie who came up with the idea of finding another host for Wanda. They didn't just grab any body, as Melanie explained. Some humans are paired with a host when they are very young and are not able to fight back like Melanie did. Or sometimes, like in the case of Kyle's wife, the Soul has been living in the body too long and the human has simply disappeared.
Melanie (from her own experience) was able to find several bodies that would have not have a human presence. Jared got to narrow it down even further to a few face that looked like they could be trusted since Wanda had always been distrusted. And Jamie got to pick the final selection because, as everyone explained, Jamie knew Wanda best and therefore knew what she should look like. Ian, it is explained, didn't care what type of body they chose since it wasn't the body that mattered to him. He did, however, help during the operation. He very tenderly tells Wanda that he held her Soul in his hands and he thinks she is beautiful.
And then at the very end Wanda, Ian, Jared, Melanie and a few others are getting ready to go out on a raid. They walk to their truck which is hidden in the desert only to find that there is a group of humans waiting for them there. The other group explains that they are also a community of survivors, that there are several other groups that share information and supplies, and they simply wanted to introduce themselves and offer any assistance they could. It turns out that this group of humans has a Soul living with them just like Wanda is now living with her family. Apparently there are a group of Souls that have "gone native" just like Wanda.
"It's a strange world, isn't it," one of their new friends asks.