X-men meets Agatha Christie. I seriously got a LOT of X-Men feels out of this book, but this is not a bad thing. I really liked this book. 
 This was a fast, enjoyable read. It's a light YA sci-fi that does not aspire to more than it is. So many books of that category I've read lately have overreached or have been overambitious in their plot and premise, this does not. The setting is contemporary and easy to understand, the concept of the Variants is well-explained and does not go into so much detail that it creates confusion and collapses within itself. There is no Bigger Message that makes me want to roll my eyes as I read; it's light fiction, and should properly be kept that way. There is an underlying plot, but it is not overreaching on a grand scale, and never overwhelms the main plotline. It is more of a setup to the next book(s) in the series, and it is so unobtrusive that I didn't mind the setup at all. Too many books try to setup the sequel to a ridiculous extent (Regency romances are particularly guilty), but this book is very restrained in building the intrigue, and I find myself looking forward to the sequel, something almost unheard of in light YA fiction these days. 
 The premise: present-day United States. The beginning of the book is set in Montana, the headquarters of the FEA, an acronym for Forces with Extraordinary Abilities. These are special people born with special powers, think X-Men mutants. Their special abilities vary, from Alec (the projected love interest) with extraordinary strength) to her friend Holly (invisibility); there are a wide, wide range of abilities, and how these abilities manifest and work is not completely understood. The author makes this clear that this concept is not fully understood by the agency or by the scientists involved in the agency itself; they just work with these special people, the technicalities are not fully understood. Thank you. I'm glad to have this out in the open instead of having a long, convoluted half-assed explanation of how these things work. Keep It Simple, Stupid. That's just fine with me. 
 Tessa is one of the Variants. Her ability is particularly promising, she can absorb other people's DNA and use it to turn into anyone she has touched. Her abilities are simply and briefly explained: "[as] far as FEA scientists could gather, my DNA incorporated foreign genetic instructions into its own unique strand as dormant DNA that could be activated when necessary," and her skill is a ridiculously useful one: as she is told by the Major in charge, "I don't have to tell you that some people in the Department of Defense and the CIA are licking their fingers at the prospect of having your talent at their disposal. [You are] the perfect spy--the ultimate weapon." 
 Tessa doesn't feel like the ultimate weapon, though. She has only been at the Institution for two years, abandoned by her father as a young toddler and by her abusive and neglectful mother, who hates Tessa's abilities. Her self-esteem is low enough for her to feel scared, lonely, and not ready for her next assignment: that of becoming Madison. 
 There's been a serial killer targeting people in Livingston, Oregon. His or her latest victim is Madison, who is about to die. Tessa's mission is to become Madison and investigate the murder as an insider. After Madison dies of her injuries early on in the book, Tessa assumes her place. Even though she has spent much of her life outside of the Institution, her life has been difficult, and she finds it hard to keep herself apart from Madison as she incorporates her life. Madison's life is so different from anything Tessa has ever known: loving parents, a caring twin brother, friends. Not only that, Madison herself has secrets of her own that Tessa gradually uncovers over the course of her investigation. Tessa knows to hold herself apart from her family, but they grow on her, and she is torn between her enjoyment of her new "family" and her guilt at deceiving them. This normal, loving family is so vastly different from her childhood. Tessa's memories of her own childhood are much more painful...her memories run along the lines of abuse: "...like the day my mother's third husband came home drunk and locked me in the closet, forcing me to listen to him beating the crap out of my mother. Or the day my mother said she wished I'd never been born," and this mission is enjoyable to her, no matter how much she tries to distance herself. 
 "I couldn't remember a time when I'd been happier. The smile seemed to be permanently carved into my face, my muscles aching from unfamiliar use. Was this how my life could have been if I had been normal? In that moment, I wished more than anything that I could keep them, that I was more than an impostor with a borrowed family." 
 Madison's body gradually becomes so familiar to her that it doesn't even feel strange to look into the mirror and see her face instead of Tessa's own, but still, deep down she knows that she is just a hollow imitation of the dead girl. I expected less complexity from her character, and was pleasantly surprised at her character development and how much I enjoyed watching her grow in such a short amount of time. Her actions are sometimes a little silly and out of character for someone trained to be a spy/assassin, but given her short time in training and her age of sixteen, her flaws can be overlooked. 
 The only character that really bugged me about this book is Alec, her friend and fellow Variant on whom she has a major crush. He is overprotective with her, having known her since she joined the Institution, but his overprotectiveness feels more like he is patronizing her. He constantly reminds her that this is a mission, he constantly dismisses her observations as false. Alex knows about her difficult past and anticipates that she will be attached to her new "family, he often reminds her needlessly "You still realize that this is a job, right? Sometimes I think you forget that." He is arrogant, condescending, and wishy-washy. He has a girlfriend, another Variant, yet freely admits his feelings for Tessa. He even acts on his feelings with Tessa, kissing her, flirting with her...while still with Kate. Not cool. And then afterwards, "It shouldn't have happened. It was a mistake." Oh hell no, that's not something a girl wants to hear. I was angry at Tessa for her constant mooning over Alec, but gradually she comes around and realizes that she deserves better than that. 
 There is a long list of very credible suspects, and I truly enjoyed playing armchair detective as I read this book. So many times, I find myself saying, in truly childish, gleeful fashion "OH, HE'S THE ONE! I KNOW IT," only to be sidetracked by yet another credible suspect later on. The plot-building and mystery goes along at a credible pace, there were no unreasonable assumptions and irrational jumps to conclusion that isn't supported by some kind of evidence or a rational assumption. I never felt that the mystery or suspects were overdone; in so many detective novels, evidence A jumps to evidence B and character C only to have the culprit be character Y because of reasons ASKFDJKSDGJ. This book is not like that. The mystery and reasoning are completely feasible. 
 I really had to giggle at the minor subplot because it was so X-Men-ish. I wanted to shout "MAGNETO" at one point reading towards the end. Lest I make the story sound too silly, it is not. It's a quick, fun read, and if you have a couple of hours to spare, I highly recommend it and I will be looking forward to the sequel.