Emily Windsnap and the Castle in the Mist

When Emily Windsnap discovers an old diamond ring during a class hunt for trinkets, how is she supposed to know that the ring is half the key to unlocking an ancient curse by Neptune himself?

Now, with the ring stuck firmly on her hand, Emily finds herself under a new curse: in just a few days, she'll cease to be half-human and half-mermaid and must say good-bye to one of her parents forever. Can she possibly find the other missing ring that will break all the curses? Is there anyone who can help her - before it's too late?