Every summer in the coastal town of Sparrow, three sisters who were drowned as witches centuries ago return from the ocean to possess three girls and lure boys to their deaths. Seventeen-year-old Penny Talbot tries to stay out of the town's deadly tradition until a mysterious boy named Bo arrives, determined to uncover the truth. As Penny gets closer to Bo, she must confront the town's dark past and her own family's connection to the haunting curse that transforms Sparrow each June.