Laia and Elias represent different classes of a colonial society--Laia is of the colonized, slave class, while Elias is of the colonizing race and in training to join the army. Elias plans to desert and abandon the Empire, hating everything it stands for, but he's given an opportunity to perhaps change it from the inside. Laia seeks to save her brother from death, but must engage in a seriously dangerous mission to do so. 
 This book is one of the best examples of dual first-person I've ever seen; the two voices were written differently enough that I was never confused about whose head we were in (unlike in Allegiant). The worldbuilding is also very well done, with lots of stuff introduced but not fully explored that leaves room for future books. 
 The plot was a bit predictable--I knew from the outset that these two characters would end up in a romantic relationship despite their interest in other people, and at no point was I at all afraid for their lives, despite the constant danger they were in. But the character development was really good, with Laia going from a weak, sniveling, not-at-all fun character to a strong, assertive one. Elias doesn't change so much as he figures out his own mind and circumstances and makes those part of his worldview and self-concept.