Nothing has been the same for Will ever since what happened last summer. One day, on an ordinary sailing trip with his brother, there is a strange accident. When Will wakes up, he learns his brother has disappeared, presumed drowned. Worst of all, Will can't remember what happened—his family finds him unconscious, with no memory of the accident. Now Will and his best friend and neighbor, Gretchen, are starting a new summer. Gretchen seems troubled—her sleepwalking habit is getting worse, and she keeps waking up closer and closer to the water. Will is drawn to Asia, the exotic new girl in town. Nobody knows where she's from—all Will knows is that her beauty and her mesmerizing voice have a powerful effect on people. Then there is another mysterious drowning, and Will and Gretchen begin to Is Asia just another beautiful, wealthy summer resident? Or is she something entirely more sinister . . . and inhuman?

At the end of Siren's Storm, the Sirens were defeated, and now the town of Walfang is once again a peaceful beach community. Or is it? Gretchen and Will are still haunted by the memories of the night the Sirens were destroyed—Gretchen because she can't remember what happened and Will because he doesn't know how to tell Gretchen what he saw. He doesn't even understand what he saw, but he does know now that Gretchen is more than what she seems, more than a human girl. And at the same time, he is more in love with her than ever. Gretchen knows there's something wrong, too. She feels like an alien in her own body, but she doesn't know why. And she feels a presence stalking her at every turn. Have the Sirens returned to Walfang? Or has some other force come to claim her?