A family secret is at the root of Mary Downing Hahn's story of supernatural events in Maine. Ali, 13, is eager to spend her vacation with Aunt Dulcie, helping to care for her little niece, Emma, in the lake house where Dulcie and Claire, Ali's mother, spent summers. Claire, who is phobic about water, is dead set against her going but is forced to agree. The vacation by the lake turns unpleasant when Ali and Emma meet a mean, spiteful kid named Sissy. Emma idolizes and imitates Sissy, becoming bratty and hostile and accepting Sissy's dangerous dares.
Sissy keeps talking about Teresa, a girl who drowned under mysterious circumstances when Claire and Dulcie were kids. At first Ali thinks Sissy is just trying to scare her with a ghost story, but soon she discovers the real reason why Sissy is so angry: she is the ghost of Teresa and blames Claire and Dulcie for her death.