After years of traveling the world, Delphine is summoned home by her grandmother to assume her rightful place in the royal House of Aegea. As heir apparent to the coveted Keeper of the Lines, Delphine is set to become one of the most powerful Mermaids in all the clans. Assuming the title, however, would mean giving up her forbidden passion for the human Max. When he decides to come looking for her, Max uncovers a hidden world of ancient magic and mythological creatures that have lived above and below the world's oceans for millennia. When an ages-old enemy returns with a powerful new weapon...