Kira has discovered that she's not human, but merrow--a mermaid--and that her biological parents are prisoners on an island run by hateful finfolk.
At thirteen years old, she's not old enough to leave her sleepy seaside town to go on a solo underwater rescue mission, so when a group of selkies offers to lead her to Hildaland, she can't refuse.
No sooner does she find the island than she is taken prisoner herself. To save herself and her parents, she has to rely on wit, her dolphin friend, Cass, and luck. But will that be enough to save the prisoners?