Briony Eddons, heir to the kingdom of Shadowmarch along with her twin brother Barrick, struggles to save the kingdom from the inhuman Qar who have begun to reach beyond the Shadowline after centuries of peace, while also trying to deal with Barrick's increasing strangeness, and other enemies from without and within.

"Rio has always dreamed of leaving the underwater city of Atlantia for life in the Above; however, when her twin sister, Bay, makes an unexpected decision, Rio is left stranded below where she must find a way to unlock the secrets of the siren voice she has long hidden and save Atlantia from destruction"--