When Neptune tells Emily and her merman father and human mother to return to Brightport to try to make merpeople and humans work more closely together, Emily faces problems with old enemies, her new, half-merfolk friend Aaron, and a mystery related to a group of legendary lost sirens.

When she incurs Neptune's wrath by finding a diamond ring buried under rocks in the ocean, Emily is put under a curse that will force her to choose to be either a mermaid or a human and split up her parents forever.

Emily is headed to a tropical island for a relaxing vacation with friends and family. And this time, Emily promises her best friend, Shona, there will be absolutely no adventure just plenty of fun. But somehow excitement always seems to find Emily, and before she knows it, she ends up on the other side of a powerful waterfall on a forgotten island no one else can get to.