1988. Charleston, South Carolina. High school sophomores Abby and Gretchen have been best friends since fourth grade. But after an evening of skinnydipping goes disastrously wrong, Gretchen begins to act -- different. She's moody. She's irritable. And bizarre incidents keep happening whenever she's nearby. Abby's investigation leads her to some startling discoveries -- and by the time their story reaches its terrifying conclusion, the fate of Abby and Gretchen will be determined by a single question: Is their friendship powerful enough to beat the devil?