Emily is delighted when her family returns to Brightport, but their attempts to follow Neptune's orders to bring humans and merfolk together seem doomed. Thrown out of mermaid school and scorned by her old enemy, Mandy Rushton, Emily goes in search of the lost sirens who were banished by Neptune to a top secret undersea cavern.

Only when she uncovers the sirens' true story can she put the pieces of her own life back together.