Disclaimer: Harper Collins provided a free digital ARC of this novel for review purposes. 
 Perry and Aria are finally reunited after months of separation, during which Perry became the new Blood Lord of the Tides and Aria was sent on a mission to find the mythical land where the Aether doesn't threaten the earth. Perry wants to go with Aria, but she knows that he can't; he is needed in the Tides and the Outsiders don't like her as it is. So she strikes off with Roar to find the one man who knows about the Still Blue--the same man who may be holding Roar's sweetheart Liv captive. 
 A lot of young adult novels take the second book to really explore the love triangle (assuming there is one) or romantic interest. This one did the opposite by splitting the two apart about a third of the way into the book and not letting them communicate at all. Rather than getting wrapped up and lost in each other to the unhealthy exclusion of every other relationship, Perry and Aria live apart, examining their relationship, feelings, and motives on their own. I thought it was brilliant and executed very well. 
 I also really enjoyed Aria and Roar's (completely platonic!) relationship. They have a beautifully deep friendship that blossoms in this book without threatening Aria's relationship with Perry. 
 Finally, I really liked what we learn about the Aether--the first book didn't give us a lot of information, but this one lays out the history and science behind the menace and shows how nearly impossible Aria's quest likely is. 
 That said, the novel is very clearly a middle novel; it doesn't wrap up any major plot points, though it does move us very well from Under the Never Sky toward Into the Still Blue. For a middle novel, it is very strong and interesting, avoiding the slump in action and characterization that usually occurs in second books in trilogies.