Dove, Luce, and Connie are best friends arguing over the new perfume Venom. The manufacturer only ran one ad that informed people it was available from Dry Ice, which is some random trendy store that sounds like Hot Topic. Since Dove is incredibly shy and quiet, they think it's ridiculous that she would even think about buying a bottle. She still checks under her bed before going to sleep and worries about everything in the world.
When they get to the store, she instantly worries about the perfume. She picks up a lingering underlying scent in the store and feels uncomfortable. Her friends open the bottle, which by the way looks like an ancient snake and not the bottle on the cover, and they don't like the smell. It appeals to Dove though, and while her friends think she's crazy, she decides to buy a bottle.
Connie and Luce drop her off at home, which turns out to be some mass produced condominium complex. She keeps thinking about the perfume to the point where she can't remember where she lives and has to look at her ID to find her address. Dove gets to her home and accidentally drops the bottle on the ground, which lets the perfume seep out. She feels a little weird but can't put her finger on it. The next day, she starts asking questions about people having two brains and what would happen if two people shared the same body. Dove worries that there really is something under her bed now and that it has taken over her body.
When she learns from her father that they originally thought they were having twins, one of whom they planned to give the unfortunate name of Wing, it all makes sense in her head. She starts thinking that Wing was there all along and that her twin sister is what now lives under her bed. Dove pretty much decides that her sister is sharing her body.
This all leads to her opening the bottle, passing out, and waking up to find that her dead twin took over. Wing now has complete control over her, while she has to just sit back in the brain and watch and listen. In history class, the teacher asks all his students to come up with a topic and write a paper on how that topic changed from ancient times to modern times. Wing suggests writing about snakes, which he turns into a joke. She grows furious and keeps saying that she'll get him back. Wing then freaks out their parents about random stuff, but it doesn't help that she and Dove keep talking to themselves and other people hear them.
Back in history class on another day, Wing and the teacher get into it again. She decides to make him breathe the perfume, which will make his dead twin come out. Dove says it's a stupid idea and explains that not everyone has dead twins and...oh my god, is this book over yet? Ugh. So, Wing makes him smell it and nothing happens except the other students complain about the smell. This cute guy, who apparently liked the real Dove, makes a joke and laughs about it. Somehow, Dove gets control over her body again, but Wing says that she will come back.
Cute guy has a name, Tim. She credits him with bringing her back because he blew her a kiss right before Wing went into hiding. They hold hands, he asks her out, and they seem like they might start dating. The whole group heads to the mall, and Connie keeps talking about Dry Ice. Turns out that the story is no longer there and that no one really remembers hearing about it. Even Luce and Dove don't say a word.
Tim asks Dove to go with him to some hot air balloon thing. Right before he picks her up, Wing takes back over. She snaps at hm for bringing her coffee and throws it out the window, talks down to him, and generally acts like a huge bitch. When they get up in the air, she threatens Dove and says she'll push him out of the balloon. She actually tries, but Dove takes over again. Tim knows something is wrong, drops her off early, and pretty much makes it clear that he has no interest in her.
This super bitchy girl named Hesta hates Dove and her friends. Wing takes over again and becomes best buddies with her. Hesta thinks it's funny the way she talks to herself like she's two different people and prefers her bitchy attitude to the way that real Dove acts. Wing goes a little crazy after seeing a glass pyramid in the mall and jumps in the water fountain. Dove says it looks like she's baptizing herself. Mall security threatens her, and she runs outside to Hesta's car.
On the way home, Dove and Wing fight over what to do. Wing wants to get back at Tim, but she also wants to kill Hesta. She contemplates yanking on the steering wheel and careening off the road. Hesta naturally hears everything and just as naturally pulls her car off the road and runs away in fear. She tells others what happened, which leads to a meeting between her, her parents, and a school counselor.
Her parents tell her that the whole Wing thing was just a story and not true. Wing naturally doesn't agree with their assessment. The counselor then gives her a shot, which knocks her out, and places her in a mental hospital. She has to go through a number of sessions with doctors who think she has multiple personality disorder. Wing refuses to say a word or come back out, claiming that since they don't believe she exists, she'll wait until the perfect moment to show she's there.
Wing then tells her that it's time for her to rest in the same way that she did for the last 15 years. Dove will still be there, but she'll be there in the back of their shared mind while Wing lives out her life. She somehow manages to fight back and gain control of her brain. Even though she's tots cray-cray, the doctors decide to send her home after just one week.
Ooh, we're so close to the end! Luce comes up to Dove/Wing and tells her that they'll always be friends and that she's there if she needs anything. Wing lets Dove know that she plans to kill Luce and all her other friends. She then heads to the mall with Hesta. Hesta wants to go in the new store that took over for Dry Ice, which now sells potpourri and other crap. Wing says it's an old lady store, which I agree with, but goes in anyway.
When she picks up the scent of lilac, she remembers that it was around during ancient times, which somehow gives her the strength to fight back. She pretty much just stats mentally pushing on her mind and telling Wing to go away. Wing whines for a few minutes and then disappears. Pretty anticlimactic. The book ends with her and Luce repairing her friendship and Dove deciding to carry lilac with her all the time to remind her of what happened. Yeah, I have no idea what the hell happened in this book either and I just read it!