After Rendell Locke is murdered at the hands of former student Sam Lesser, his wife Nina decides to move with her three children, Tyler, Kinsey, and Bode, from Seattle to Matheson, Massachusetts, and take residence in Rendell's family home, Keyhouse. The children soon discover a number of mysterious keys throughout the house that can be used to unlock various doors in magical ways. They soon become aware of a demonic entity that is also searching for the keys for its own malevolent purposes.