Imprisoned in the heart of a secret military base, Em has nothing except Finn's voice coming from the cell next door and the list of instructions she finds taped inside the drain. Only Em can complete the final instruction. She's tried everything to prevent the creation of a time machine that will tear the world apart. She holds the proof: a list she has never seen before, written in her own hand. Each failed attempt in the past has led her to the same terrible present, imprisoned and tortured by a sadistic man called the doctor while war rages outside. Marina has loved her best friend James since the day he moved next door when they were children. A gorgeous, introverted science prodigy from one of America's most famous families, James finally seems to be seeing Marina in a new way, too. But on one disastrous night, his life crumbles apart, and with it, Marina's hopes for their future. Now someone is trying to kill him. Marina will protect James, no matter what. Even if it means opening her eyes to a truth so terrible that she may not survive it. At least not as the girl she once was.All Our Yesterdays is thrilling, complex, and compelling, a story of making difficult choices in the name of exposing the truth and survival. Dual point of view introduce readers to to very different girls, two girls who are more connected than is expected. Their lives, their timelines, are about to intersect and reveal a surprising and agonizing truth.Em is imprisoned, trapped. She needs to break free. Em seems to be fully realized as a character, she knows who she is, she knows what has to be done, and she knows it's going to be almost impossible and utterly painful. She has the feel of an end result as a character. Her world is dark, painful, and not what it should be. A number of things are unsaid at the beginning between her and Finn, but what's clear is that they both need to travel back in time to stop something from happening, to stop someone from becoming a monster. To return the world around them to what it once was.By contrast, Marina hasn't become her own person yet, she isn't as solid. What she is is intelligent, caring, and harbouring a puppy-love crush over her neighbour James. When his life crumbles she jumps right in as someone for him to lean on, as someone to keep him safe. But there's a lot Marina doesn't know that could change her mind about recent events, there are people in her life she doesn't know the truth about. And she doesn't know Em is on her way.The time travel aspect is treated well, it's roots are in science and deals with complicated equations regarding different dimensions, but it's not overused or abused in any way. Em and Finn travel back in time, into Marina's timeline, to make the one choice that will keep her world from becoming theirs. But can the past be changed?For most if not all of the book, the focus is on trust and choice. How to we decide who to trust? Is it enough to trust their words, or must we see it for ourselves? What if we make the wrong choice? What if we could go back and not make that choice, knowing what it leads to? Yes, this has time travel, but it's less about the science of it and more about the choosing to go back, more about the moral implications and the repercussions, more about the human side of it. Every time Em went back she was hoping to make the right choice, hoping that everything would work out, hoping that Marina would get the chance to live.I found this to be such an intriguing story. Given the chance, would we go back and right wrongs, keep the world from changing in certain ways, keep those we care about from leaving? Should we go back? What if those changes are dangerous? What if they require action to be taken on the biggest scale? What if they mean murder? What if realizing the most painful truth means making the most painful sacrifice? Just before I started this book, I discovered it's actually the first in a duology. Taking that into account, along with the abrupt but powerful ending, I'm a little confused but also curious about the next book.