Olivia and Olive are mermaid twins abandoned in the human world just after birth at a place called Leamouth, on the Irish shore. They are adopted by two different young couples. Olivia is raised in Florida, and Olive is raised in California. At age 14, they meet again in Leamouth. They discover a secret lake inside a cave, and when a lunar eclipse takes place, they sea claims them again. The ocean starts calling to them at night, and when they confess this to each other, they decide to answer the call together. Soon, they realize who they are, and they embark on a quest to find their true family.
In the epic conclusion to the Devil's Deal series, Freya has one final chance to discover the full potential of her own power, and to use it to restore balance in the universe - once and for all. After the destruction caused by Roisin, Ferya and Torix come to terms with their losses and mistakes... And prepare for a final battle that will change everything.