Olivia is curious about the people moving into 16 Olcott Place. The last family there moved out in the dead of night, and the new family, the Donahues, has no idea why. Olivia becomes fast friends with Janie Donahue . . . so she's there at the house when the first of the letters arrives:

--I am the Sentry of Glennon Heights. Long ago I claimed 16 Olcott Place as levy for my guardianship. The walls will not tolerate your trespass. The ceilings will bleed and the windows will shatter. If you do not cease your intrusion, the rooms will soon smell of corpses.--

Who is the Sentry? And why does the Sentry want the Donahues out of the house badly enough to kill? As Olivia and Janie explore the house, they find a number of sinister secrets . . . and as they explore their town, they find a hidden history that the Sentry wants to remain hidden forever.