For Mer handler Finn Jarvis, the feral mermaid performers at Oceanica Marine Park are nothing more than ruthless aquatic predators, violent and unpredictable. That doesn't stop the public from flocking to one of the world's most popular tourist attractions. To them, the Mer are magical entertainment, too beautiful to be dangerous. They don't see what happens to the ones who dare swim out of line.

Newly caught Erie doesn't know what the evil landfolk want from her. Alone and voiceless, she watches the other merfolk from the confines of her tank. Broken into submission, they've become shells of the once vibrant creatures she knew. But Erie refuses to be subjugated. She'll get the crowds to see her as something more than simple entertainment—starting with her captor, Finn.

While Finn trains Erie in her routine, she secretly teaches herself the air-words he and the other trainers speak. And when the language barrier falls, Finn starts to realize that the gap between human and Mer is smaller than he thought, and maybe it's not the Mer who are monsters, after all.