A fast-paced fantasy adventure tale in a richly drawn dystopian future. Despite her lifelong dream of living Above, recently orphaned Rio has promised her twin sister, Bay, that she'll stay in their underwater city of Atlantia when they come of age. In one shocking moment, however, Bay is headed Above, and Rio is left alone, separated from the last person who knew the secret of her hidden siren voice and loved her anyway. As Rio tries to find her own way to get Above, she also discovers pieces of Atlantia's hidden past and its uncertain future. Ultimately, the protagonist will have to rethink everything she's been taught and make courageous decisions on her own in order to reunite with her sister and save the world she loves. Complex characters, including Rio's antihero aunt, and a realistically slow and subtle first romance make this a book teens will relate to, even non-genre fans. A slowly unfolding backstory perfectly complements all the action. Despite a bit of a didactic lean in the final chapters, this is a title that's sure to be immensely popular with teens, especially those who enjoyed Condie's "Matched" trilogy.