Teenager Olivia Danvers knows that the previous residents at 16 Olcott Place moved out in the middle of the night, but nobody really knows why; the new family, the Donahues, have three children including a daughter, Janie, Olivia's age, and the two become close friends--but when the threatening letters start arriving from the "Sentry of Glennon Heights" the two girls realize that the house and town are hiding sinister secrets, which could tear their whole world apart.