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 Emily provokes Neptune's wrath by finding a diamond ring buried under rocks in the ocean, he puts her under a curse that forces her to choose between being a mermaid or a human. When Emily provokes Neptune's wrath by finding a diamond ring buried under rocks in the ocean, he puts her under a curse that forces her to choose between being a mermaid and a human.

For as long as she can remember, twelve-year-old Emily Windsnap has lived on a boat. And, oddly enough, for just as long, her mother has anxiously kept Emily away from the water. But when Mom finally agrees to let her take swimming lessons, Emily makes a startling discovery -- about her own identity, the mysterious father she's never met, and the thrilling possibilities and perils shimmering deep below the water's surface. In a lovely twentieth-anniversary...