Kira has discovered that she's not human, but merrow - a mermaid - and her biological parents are prisoners on Hildaland, a mysterious fog-bound island run by the hateful finfolk. Only thirteen, she's not old enough to leave her sleepy seaside town to go on a solo underwater rescue mission. But being resourceful and determined she searches out old Babbling Bill to learn more about the mythical selkies. Do they really exist, and if so, would they be willing to help Kira find Hildaland? If she gets that far, can she evade capture by the finfolk? Once again Kira must face dangers with her quick mind and occasional help from her undersea friends. But will those be enough to save the prisoners of Hildaland?