Waverly and Kieran are together again on the Empyrean. Now that the girls are back, the mission before them seems slightly less impossible: catch the New Horizon and get their parents back. But nothing is as it seems, not anymore. Kieran's actions have raises Seth's hackles, and quite possibly Waverly's. Seeing the change in Kieran, she finds herself more and more aligned with Seth. In one night, an explosion rocks the ship, sending them off course, and somehow, Seth is freed from jail. He becomes the most obvious suspect for the explosion, as Waverly becomes the most obvious suspect for releasing him. Tensions slowly build in this ship of frightened children and teenagers grasping to hold everything together, but soon they will reach the boiling point. Will the culprit be found? The balance of power in the Empyrean is precarious, and the clock is ticking.Spark is just as fast-paced as Glow, just as isolating and dark and mysterious, just as dangerous, and packed with even more suspicions. This is the kind of science fiction thriller that will draw in teens as well as adults, never letting up, only giving glimpses of hope and quiet before the stakes are raised even higher than before.The Empyrean has changed so much since the beginning of Glow. With few adults and lots of untrained children at the helm, everyone who thinks they know what's best reach out to grab hold of the rest, to be the one in charge. This series is like a tense thriller that just happens to take place in space. Speculation and double-talk, suspicions and danger, scapegoats, hiding the truth with lies, hiding lies with the truth, picking sides. And all the tension is amped up because this is all happening in a crippled spaceship in the vacuum of space, so you never know if your next breath will be your last.Waverly comes back damaged, in more ways than one, and is left with little to sustain her beyond her hatred and fury towards the New Horizon. It seems like there's nothing left that will make her happy, not with her mother stuck on the New Horizon and Kieran becoming someone she no longer recognizes.I still ended up with a massive sense of uneasiness after reading this book. The first left me with all kind of weird feelings (plus a massive sense of relief that I don't live on either of the ships). There are such high stakes, such brutal treatment, such a sense of confusion and loss and unknowing what will come next and not knowing who to trust. There's little to no trust in this series because you never know who to trust, not unless you read about Waverly's or Kieran's or Seth's actions for yourself. Everything else is up for grabs, and you have to hope that you will survive in the end.