"Astrid struggles with a shameful secret, Ling discovers courage in a prison camp, and Serafina lines up allies to help fight the final battle for her underwater realm"--
"Astrid must confront her ancestor, Orfeo, the evil force behind the rise of the monster Abbadon, while Sera and her Black Fins train goblin troops for battle against her uncle Vallerio's death riders"--
Uncovering an ancient evil, Serafina, a mermaid of the Mediterranean Sea, searches for five other mermaid heroines who are scattered across the six seas, to save their hidden world.