Actual rating: 1.5, rounded down for the slut-shaming. 
 Writing science fiction based on actual science or physical theories is a tricky thing. More often than not, unless you have an absolutely rock-solid premise and your plot is firmly based on said scientific theories, plot holes will emerge, and ultimately the faulty premise on which the pseudo-scientific plot is based will come back to bite you firmly in the ass. Add that to extreme slut-shaming and you have the formula for a book I wish I had not read. 
 I have a high respect for the author for even attempting an ambitious topic such as this. But the author's credentials is that of a writer; she is not a scientist. It shows in this book. 
 I am not a scientist myself, but I am a fan of it. I am a scientific dillettante. Esteemed physicists like Michio Kaku and Neil DeGrasse Tyson make string theory, extradimensional science understandable for the layman. This book is not to be used as a reference in any way, shape or form for anyone remotely interested in science. It is supposedly based on actual science, the words Einstein, dimensions, string theory, etc. are tossed around like candy strewn from a pinata. E=mc2, W=VxT, parallel dimensions, etc. are mentioned. They are meaningless. The underlying plot in this book is based around flimsy pseudo-scientific theory that the author has bent to fit her plot, not the other way around. 
 The science does not work with the story, the story bends the science to fit its needs. A good book on parallel dimensions and string theory is hard to find; in some cases, it can be well-done. The problem is that this book goes into too much details and therefore created more questions than it was able to answer. If you are throwing parallel dimensions out there, and explaining it in details, you had better be prepared for the possibilities. Saying that every decision made creates a parallel dimensions is well and good, that may be true. But here comes the plot holes (extradimensional worm holes?). If there are so many dimensions, who's to say which is the real one? How did they know to find each other? We are not talking about one person for each possible dimensions, even so, there is a infinite possibility because we make so many choices every day. Who's to say which decision will split into a different dimension? Who's to say how many different parallel universe there exists out there? Who's to say that the main character in this particular universe is the main character? What makes her so special from all the other Lillies from all the other infinities of existing dimensions? And yet this is the premise of the book. 
 The first half was a cliche of high school drama, filled with overwrought high school stereotypes. The nerds, the wealthy bitches, the queen bee, the jocks, the sycophants. Name a high school trope, I will find them in this parade of pre-cut characters. The second half is a painful mess as the author remembers that there's supposed to be a plot beyond the Mean Girls setup and struggles to weave in the actual sci-fi stuff because that's actually the backbone of the book. 
 I wanted to read this book for two reasons: 
 1. The cover: it is a pretty cover, it is a mysterious cover. Anything with a key on a red ribbon on it just screams "mystery" and "UNE GRANDE AFFAIRE D'AMOUR" to me, because as much as I pretend to be a mature adult, there's a 13-year old lurking in the deepest corner of my psyche who just wants a legendary love story. With added secrecy. Because that's what the key not-so-subtly implies. Judging a book by the cover? It's not just a proverb for me, because I am easily distracted by shiny things. 
 2. The summary: the very, very vague summary that hints of a possibly interesting Big Reveal. I had no idea what this book was about. The vagueness of the premise added to the mystery of this book, and gosh darn it, I wanted to find out. 
 Sadly, the book did not work for me. It started off well-enough, and I loved the writing style initially. Lillie and her friends, Sylv and Jo seemed like your typical teenagers. Dorky, geeky, quirky. Each has a very distinct personality of her own, and I could very realistically imagine that these could be actual characters that I could know in real life. I loved the description of the small town of Green Grove, the tiny little backwaters town where everyone knows everyone, the butcher, the baker, the candlestick maker. Where newcomers are a rare treat, and like our love interest Tom, treated like a mythical Loch Ness monster until they actually emerge. I love the little details and commentary of the town and its people. Even Deb, Lillie's "mother" only in the very faintest sense of the word, is initially an interesting character, and I thought I could see her potential growth into a mature woman who actually has a relationship with her daughter. 
 I had a lot of expectations that had every possibility of being fulfilled from the extremely promising first three chapter. I thought I could see where the characters would grow, how their characters would develop gradually into complexity, from immature teenagers with silly dreams into realistically flawed characters, wiser from their experience. But then I read on, and my hopes were destroyed. Everything ended with the entrance of the Loch Ness Monster. Tom. 
 From then on, it was an excruciatingly painful downhill slide complete with pot holes (plot holes?) and jutting rocks where I never expected there to be. 
 Lillie: initially sympathetic, average small town girl with an artistic bent. Falls into horrifying insta-love with new boy, feels like she's known him her whole life. Is shocked and dismayed despite her rational sense when he doesn't return her feelings. She behaves idiotically, contrarily. She makes excuses for someone who could have killed her, because HE DIDN'T MEAN IT. Yes, it's so easy to forgive someone who places you within (literally) a microsecond of death and dismemberment. But no, she's not such a martyr. Lillie becomes angry at the Love of Her Life (although he doesn't know it yet) for saying her photographs were overexposed (they really were). 
 Tom: I haven't much to say about him. He's your typical hot-and-cold love interest. He smiles at her one moment (and the sun broke out of the sky, the birds sang, a double rainbow forms), and ignores her for the queen-bitch Melissa in the next. He says cryptic things, hints at a Mysterious Past, etc. Nothing new here. It's just his Big Reveal that has my mind completely boggled, and not in a good way. Personality-wise, Tom is no better or worse than the majority of your YA love interests. 
 And here we get to the part that makes my blood boils. Slut shaming. Lots of it. There is no such thing as slut-shaming a girl by accident in a book. It is done deliberately, provocatively. I would say this book is one of the worst I have read when it comes to making snide comments about girls who dress provocatively and act sexually precocious. Other books piss me off because the main character refers to a girl she hates as a slut, a whore, etc. This book is worse because it is done so underhandedly. 
 Sylv is one of Lillie's best friend. Sylv is an aspiring model, she is sexually outgoing. Lillie constantly makes offhanded remarks in her narrative about Sylv's looks, the way she dresses, the way her underwear shows, the way she sits with her crotch hanging out for the world to see, the way she flirts, makes obscene and crude gestures towards older boys, the way she turns everything into a sexual metaphor, the way she refers to a love story as a "porno," etc. It is as if the author goes out of her way to make her disapproval of the character known, and I absolute hated it. It is constant, it is pervasive, it is repetitive. Sylv is rarely called an actual SLUT by her friends, but the sneakiness of the commentary and the sly backhanded tone of disapproval makes me furious. This behavior of hers continues throughout the book, and it was so upsetting to me. 
 Furthermore, other characters are portrayed in the same way. And what's worse, it is the other girls. Every single girl in this book, Lillie's best friends included, are portrayed as bad. I hate it when a book rips other girls apart at the expense of the main character. From the insecure fat best friend Jo, who acts out and does something sexually taboo, to the queen bee Melissa (Tom's would-be seducer) who dresses like a streetwalker, complete with copious amounts of self-tanner. This book is a portrayal of the worst of female stereotypes, that women hate each other; I, for one, am sick of it. 
 I received a copy of this book for review from Netgalley.