Cobweb

Eight-year-old Peter hears scratching sounds from inside the walls of his house at night. His parents, Carol and Mark, tell him it is nothing — first dismissively, then with an urgency that makes the dismissal feel like something else. At school, a substitute teacher notices bruises and the particular wariness in how Peter holds himself and begins asking questions that the school does not want asked.

The voice in the wall is a girl's voice. She tells Peter her name is Sarah. She says she needs his help. Peter's parents discover he has been listening to her and their response is violent enough that he stops asking about her where they can hear. He keeps listening.

The substitute teacher visits the house. Peter's parents kill her. Peter finds out what his parents have been concealing behind the wall: Sarah is his older sister, kept imprisoned in the house's hidden spaces since before he was born. She has survived in the walls and crawlspaces for years, feral and warped by confinement, subsisting in the dark.

Sarah breaks free. She kills both parents. She is not the rescue Peter needed and not the sister he imagined — years of what was done to her have made her something dangerous. Peter survives. The film ends on the two of them, the house quiet around them, and the question of what happens to a child who was created to replace the one that was hidden.