Emily Windsnap has never been in water before, and during swimming lessons she finds out why. While swimming she gets a weird stiff feeling in her legs, and feels like her legs are stuck together and quickly gets out of the pool. Later that night she goes swimming again and stays in the water and realizes she is half mermaid! When Emily learns about her past she finds out her dad (a merman) was imprisoned for marrying her mom (a human) because it was against Neptune's (king of the sea) law. As she and her new friend Shona set out to rescue Emily's dad they realize it's not as easy as it seems,then they get captured by Neptune! What will happen to Emily?