After learning that they aren't human, Cassie, Diana, and Thomas are officially accepted into the magical world that Dillon has always known. Thomas begins training to be the first mermaid seer by Elsie, a seer whose last assignment failed miserably. Diana enters the Fairy Academy to learn about her own magic. Dillon and Cassie, on the other hand, search for more clues about their prophecy and how to defeat Zared. During their search, they find that the warlocks have been brewing a powerful potion that has been kept a secret for centuries. Soon, the five friends figure out that the potion may require an ingredient that the sirens possess. Ignoring her friends' advice, Cassie decides to go into the ocean alone to figure out what the ingredient is and, just maybe, what the potion can do. After succeeding, however, she is kidnapped by an inside spy bent on revenge. As Cassie struggles to survive in Zared's domain, Dillon, Thomas, Diana, and Elsie race against the clock. Because Elsie knows that it will take all four of them to save her...