It's been thirty days, two hours, and seventeen minutes since Calder left Lily standing on the shores of Lake Superior. Not that she's counting. And when Calder does return, it's not quite the reunion Lily hoped for. Especially after she lets her father in on a huge secret: he, like Calder, is a merman. Obsessed with his new identity, Lily's dad monopolizes Calder's time as the two of them spend every day in the water, leaving Lily behind.

Then dead bodies start washing ashore. Calder blames his mermaid sisters, but Lily fears her father has embraced the merman's natural need to kill. As the body count grows, everyone is pointing fingers. Lily doesn't know what to believe—only that whoever's responsible is sure to strike again.