Also posted on my book blog, Rinn Reads.From a non-linear, non-subjective view point, time is not simple. No, in fact it's more...Thank you, Doctor*. Bearing that in mind, books about time travel are never going to be simple. It is something that has fascinated humans for a long time, the possibilities it would bring but also the potential troubles it would cause. And when I heard that this book, so loved by many a YA fan, was about time travel, I was quite surprised.That was, until I read it myself.Because it's not simply a story about time travel. Whilst that is the main essence of the story, the focus of the plot, it's so much more. It's also a story of young love, of heartbreak, friendships, power, intelligence. The story is so well-structured; the flitting between past, present and future works so well and builds up to the final events perfectly. From the very first chapter, I knew I had to find out what was really going on, and soon. As if that wasn't enough, a couple of chapters in one little word had me reeling, needing to know more. The plot is exciting, the action fast-paced and exhilarating and there are several little twists thrown in to catch you off guard. Although I guessed one important element of the story early on, it was revealed shortly afterwards and is a crucial plot point from thereon out.The time travel in this book is explained early on, conveyed through exposition, and best of all it's not unnecessarily complicated. I'm not particularly scientifically-minded, so I don't know how it would sound to someone who is, but it made sense to me - at least in the context of the story and its universe. With well fleshed out characters and relationships, built up more effectively by 'flashes' - moments where a character blacks out and relives a past moment, caused by the time travel - Cristin Terrill cleverly makes you both like and dislike the same character through the use of different points of view. And that's what makes this such a tense read, because you're at once both aching for someone to get their comeuppance and simultaneously be saved. It also meant that I wasn't always rooting for Em and Finn's success in their mission, and at other times I was.Marina, one of the main female protagonists, was snobby and rather selfish, as well as being a bit of a spoilt rich kid. But I couldn't dislike her completely, I felt that her lack of parental contact redeemed her from that a little, and she had a certain naivety to her that couldn't be ignored. There was a great contrast during the first third or so of the book, between Marina's normal life, where her biggest worry was confessing her feelings to James, and that of Em, imprisoned, tortured and with a secretive and tangled past. Em is tough and hardened, mysterious and brave.And as for the guys... it was nice that they weren't too different, which is how it often seems to be. Often they have opposite colourings, interests, body types, personalities... but both boys are intelligent and ultimately well-meaning. James is so brilliant and full of good potential but... it's hard to write much about the characters of this book without getting a bit spoilerific.Take it from someone who knows all about spoilers.It's not often a book has me totally torn between characters and events. It is powerful, drawing you in and making you unsure of the conclusion you wish for. The ending was heart-breaking but with a sequel on the way, things might not be as they seem... and I'm fine with that.I like GIFs now. GIFs are cool.*Disclaimer: I am not sorry in the slightest for the Doctor Who references.