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.

The New York Times best-selling series continues with the half-mermaid embarking on an icy and perilous journey to the Land of the Midnight Sun.Strange storms are blowing up from the ocean, caused by the nightmares of Neptune, King of the Sea. Convinced that his dreams herald an approaching threat, Neptune enlists half-mermaids Emily Windsnap and Aaron for a top-secret mission to find and eliminate the source of the trouble. But Emily and Aaron quickly...

After finally convincing her mother that she should take swimming lessons, twelve-year-old Emily discovers a terrible and wonderful secret about herself that opens up a whole new world.