Eight-year-old Peter lives in an isolated suburban home with his overprotective parents, Carol and Mark. Peter is an outcast at school, constantly tormented by an aggressive bully named Brian. At home, Peter is awakened late at night by persistent knocking sounds coming from inside his bedroom wall. When he tells his parents, they dismiss his concerns as an overactive imagination and scold him for disturbing their sleep.

The tapping continues, eventually accompanied by a girl's soft, raspy whisper calling out to Peter from behind the plaster. The voice claims she is his older sister, locked away inside the walls by their parents because they consider her a monster. At school, Peter draws a disturbing picture of a boy crying out for help in a pitch-black bedroom. His substitute teacher, Miss Devine, spots the drawing and visits the house out of concern, but Carol coldly rebuffs her, insisting Peter is prone to tall tales.

As the voice in the wall continues to speak, it warns Peter that his parents are planning to kill him just as they attempted to kill her. Mark and Carol's behavior grows overtly sinister: Mark forces Peter to stand in the dark basement as punishment for speaking out, and Carol flies into hysterical rages. When Brian destroys Peter's hand-carved pumpkin at school, Peter retaliates by pushing Brian down a flight of concrete stairs, shattering his leg. Peter is expelled, and his furious parents drag him into the basement, locking the cellar door and trapping him in darkness.

The voice directs Peter to a hidden grate in the basement floor, through which she shares food and builds trust. She instructs Peter to slip rat poison from the gardening shed into his parents' dinner to protect himself. Peter cooks French onion soup, laces it with poison, and serves it. Carol and Mark eat the soup; Carol immediately begins vomiting blood and foaming at the mouth, while Mark collapses in agony. Mark uses his dying breath to warn Peter that the thing behind the wall is evil and must never be freed.

Panicked, Peter takes the kitchen cleaver, climbs to his bedroom, and chops through the wall panel to free his sister. Out crawls a horrifying, pale, feral creature with wild, elongated hair, sharp teeth, and disjointed spider-like limbs. At that moment, Brian and three teenage accomplices break into the house wearing Halloween masks to take violent revenge on Peter for the staircase push.

The feral sister, Sarah, stalks through the shadows and brutally slaughters the intruders, ripping limbs apart, crushing heads against walls, and tearing Brian to shreds. Peter realizes that Sarah was not an innocent victim; born with severe mutations and savage cannibalistic instincts, she was walled away by their parents to protect the neighborhood. Sarah turns her rage on Peter, pursuing him through the house. Miss Devine arrives just in time to help, and together they lure Sarah into the basement furnace room, trapping her inside the reinforced pit and locking the steel gate.

With Sarah locked beneath the floorboards, Miss Devine wraps a traumatized Peter in a blanket and holds him on the front porch as police sirens echo in the distance.

In the final chilling beat, Sarah's raspy voice whispers out of the shadows directly into Peter's mind. She promises that no lock will hold her forever, warning Peter that she will always be waiting in the dark corners of his walls and will eventually return to finish what she started.