Actual rating: 2.5 
 I've read my share of books about parallel universes this year. Most of the time, the basis is simple, but then eventually the idea turns into a mess beyond comprehension, both plot-wise and conceptually. This book executes the idea much better than most of the books with a similar premise...but considering those book were mostly utter crap, this book will be elevated by me beyond that, to the glorious category of "unstinky-poo." The plot is also plagued with one gigantic, gaping, Grand Canyon-sized plot hole crucial to the twist, that I am frankly shocked that it got passed through (no spoilers). 
 The idea of the parallel universes were well-explained, but the plot and the characters leave much to be desired. Here we have the sci-fi version of The Parent Trap. Josie Byrne is a brilliant student, she's got a wonderful boyfriend and a group of friends whom she largely ignores because her family life is a mess right now (mom is acting like a mad scientist, dad is dating her school's former prom queen-cum personal-trainer, etc). Also, there's been a lot of mysterious deaths in the woods near town. All caused by animals, according to the authorities. Like anyone's going to believe that. 
 In one Very Bad Afternoon, Josie gets into a quasi-car accident, gets fired from her job, and finds out that her wonderful-awesome-handsome-loving boyfriend Nick has been cheating on her. With her best friend, Madison. Ouch. All this started from that unfortunate car event at 3:59 PM. 
 From then on, Josie has dreams of someone who looks like her, named Jo. Jo looks like she has the perfect life, because you can absolutely glean all that from glimpses of someone's dreams. Jo drives a sweet BMW. Her boyfriend (also Nick) is loving and wonderful. Jo's got a lovely bedroom. 
 Josie and Jo come to realize that the mirror in their room is connecting them and their parallel worlds, which is separated by 12 hours. Josie is desperately envious of Jo. Specifically, she is envious of Jo's boyfriend, the cheating son-of-a-bitch Nick, whom she still loves and with whom she still wants a second chance. So Josie made the brilliant idea of switching places with Jo for one day, despite knowing nothing of her besides for one briefly exchanged letter, passed through the mirror, despite knowing nothing at all about her world, all for the sake of having ONE MORE PWECIOUS DAY WITH HER BELOVED NICK TO MAKE THINGS RIGHT. 
 What?! Why?! It's a parallel universe! It changes nothing in her original world! She could be with Nick for one perfect day, he's still not going to have an idea of what transpired in her world. Josie, you are a brilliant high school student, why are you acting like such a dumb chick?! It makes no sense to me that Josie made that decision. Josie is not stupid. She really, truly is brilliant. From her demonstrated knowledge and discussions of advanced physical theories like the Penrose Interpretation, from her mother's reputation as a brilliant scientist (she's working on some experiment involving deuterium), I really, truly expected more from Josie than the behaviors of a lovesick girl. 
 Once the two girls do their little switcheroo (the normally frumpy Josie even gets her hair highlighted to match), Josie finds out that Jo's seemingly perfect life is not what it seems. Sure, she gets a sweet car, and she's rich, and popular...but her newfound popularity doesn't feel quite right. 
 It was kind of like being the world's most popular leper---the focus of everyone's attention and no one's affection---and it made her incredibly uncomfortable. Was that what it was like to be popular? A kind of cold, fearful isolation? 
 The parallel world is like hers, but with different variations, a darker feel to it. 
 In house after house, blinds were being drawn, shutters latched, like every household was hunkering down for the night. As Josie tramped along, she got a creeping feeling up the back of her spine. The entire town had an air of hostility. 
 The people are still the same, but they act differently...or maybe it's because Jo is not who Josie thought she was. Her best friend Madison hates her, the whole school is scared of her, the teachers react in shock when she answers a question in class. Her other friend, Penelope, is absolutely terrified of her. 
 And what's with this thing called The Grid? What the hell is a Nox? 
 The plot is interesting, the execution...not so great. The romance is so ridiculous, so utterly forced. It is as if Josie falls into insta-love with Nick, someone she already knew, except...he's different. And thinking about romance in this sort of situation is just plain stupid. The plot is complicated. Josie is trying to get back to her world, where the insidious bitchy Jo has taken over her life. There's the mystery of her disappeared mother, there's strange, mysterious, bloodsucking creatures flying around in the dark. There are government plots. People dying. Constant fear of the dark. It is a shitty, shitty world, a hell of a parallel universe, and yet Josie can't help but want to stay in it...ALL FOR THE SAKE OF NICK. 
 Screw the Nox. She could stay here, with Nick, forever. No one would know. She leaned her body closer to his, and closed her eyes. 
 This wasn't her world. This wasn't her life. And yet for some reason the idea of leaving here--of leaving him--made Josie instantly sick to her stomach. 
 Josie realizes the ridiculousness of her feelings...and she clings onto them still. It was so utterly disappointing. I wanted a strong heroine, instead, I got a strong heroine who was weakened; personality and strength and rationality set aside for the sake of romance. 
 The plot is heavily reliant upon deus ex machina. People just happened to be there at the right time to save someone from a bitter fate. Too many time, it happened. Oh, and this little exchange between Josie and her doppelganger father is just lovely. 
 "What do you need me to do?" Mr. Byrne said at last. 
 "We need to use the X-FEL laser. The one up at Fort Meade." 
 "When?" Mr. Byrne asked. 
 "As soon as possible." 
 She heard Mr. Byrne let out a slow breath. "All right, princess. Whatever you need. I can probably get them access for a few hours." 
 CAN YOU STEAL A HIGHLY DANGEROUS LASER FOR ME, DADDY? OK BABY PRINCESS. 
 All of the characters were just...average. They didn't have any depth to them, even Josie, who should have been developing in complexity throughout the book. Nope. Josie is pretty much the same lovesick character she was at the end of the book as when she started. She's supposed to have developed strength somewhere along the way, but I really did not see that complexity built throughout the book. The side characters are more interesting than Josie, particularly the parallel-universe version of Madison, but we never got to see more of them than necessary, because Josie has to be the star of the show, and she stays that way to the detriment of the rest of the characters. 
 I never felt like Nick was anything other than a generic nice guy, and I can see how Josie is attracted to him---but I don't see how she's heads-over-heels in love with him---in both worlds, with both versions. One version of Nick supposed to be superior to the other, but considering the standard Nick was a cheating asshole, it doesn't take much to raise the bar. 
 A promising book, built on solid scientific theories and a well-executed premise of a parallel universe that was ultimately let down by an absurd plot and no character development.