On Lily Sanderson's eighteenth birthday she'll become just a girl—still a mergirl, true, but signing the renunciation will ink Princess Waterlily of Thalassinia out of existence. That leaves plain-old Lily living on land, dating the boy she loves, and trying to master this being-human thing once and for all.

Now that Lily and Quince are together, mer bond or not, she's almost content to give up her place in the royal succession of Thalassinia. But just when she thinks she has everything figured out, the waves start to get rough. Lily's father sends a certain whirlpool-stirring cousin, Doe, to stay with her on land. And why why why is she batting her eyelashes at Lily's former crush, Brody?

The seafoam on the raging surf comes when a merboy from Lily's past shows up—Tellin asks Lily for something that clouds her view of the horizon. There's a future with Quince on land, her loyalty to the kingdom in the sea, and Lily tossing on the waves in the middle. Will she find a way to reconcile her love, her duty, and her own dreams?