Seventeen-year-old Vanessa Sands is afraid of everything - the dark, heights, the ocean - but her fearless older sister, Justine, has always been there to coach her through every challenge. That is, until Justine goes cliff diving one night near the family's vacation house in Winter Harbor, Maine, and her lifeless body washes up on shore the next day.

Vanessa's parents want to work through the tragedy by returning to their everyday lives back in Boston, but Vanessa can't help feeling that her sister's death was more than an accident. After discovering that Justine never applied to colleges, and that she was secretly in a relationship with longtime family friend Caleb Carmichael, Vanessa returns to Winter Harbor to seek some answers.

But when Vanessa learns that Caleb has been missing since Justine's death, she and Caleb's older brother, Simon, join forces to try to find him, and in the process, their childhood friendship blossoms into something more.

Soon it's not just Vanessa who is afraid. All of Winter Harbor is abuzz with anxiety when another body washes ashore, and panic sets in when the small town becomes home to a string of fatal, water-related accidents... in which all the victims are found grinning from ear to ear.

As Vanessa and Simon probe further into the connections between Justine's death and the sudden rash of creepy drownings, Vanessa uncovers a secret that threatens her new romance - and that will change her life forever.