Lily Sanderson has a secret, and it's not that she has a huge crush on gorgeous swimming god Brody Bennett, who makes her heart beat flipper-fast. Unrequited love is hard enough when you're a normal teenage girl, but when you're half human, half mermaid, like Lily, there's no such thing as a simple crush.

Lily's mermaid identity is a secret that can't get out, since she's not just any mermaid—she's a Thalassinian princess. When Lily found out three years ago that her mother was actually a human, she finally realized why she didn't feel quite at home in Thalassinia, and she's been living on land and going to Seaview High School ever since, hoping to find where she truly belongs. Sure, land has its problems—like her obnoxious biker-boy neighbor, Quince Fletcher—but it has that one major perk: Brody. The problem is, mermaids aren't really the casual dating type—the instant they "bond," it's for life.

When Lily's attempt to win Brody's love leads to a tsunami-sized case of mistaken identity, she is in for a tidal wave of relationship drama, and she finds out, quick as a tailfin flick, that happily ever after never sails quite as smoothly as you planned.