Peter is an eight-year-old boy who lives with his overbearing and emotionally distant parents, Carol and Mark. Struggling with bullying at school, Peter finds himself isolated, lacking both friends and a social life. One fateful day, he chooses to stay inside during recess, which draws the sympathy of his substitute teacher, Miss Devine. Despite his desire to go trick-or-treating on Halloween Night, Peter’s plans are thwarted by his parents’ strict prohibition, stemming from concerns linked to the mysterious disappearance of a young girl years earlier.
Later that night, while resting in his room, Peter hears a strange tapping sound emitting from within the walls. When he seeks reassurance, both Carol and Mark dismiss his claims, insisting that he is merely imagining things. Undeterred, Peter continues to interact with the enigmatic entity he later learns is named Sarah. She reveals to him that she has been trapped in the walls by Peter's parents, who she calls “evil,” leaving him increasingly unsettled.
When Miss Devine discovers a drawing by Peter depicting Sarah pleading for assistance in a dark chamber, she grows increasingly alarmed. After discussing her concerns with Peter's parents, they dismiss her worries. In a confrontation, Carol chastises Peter for exposing his “delusions” to others. Meanwhile, Sarah continues to insist that Peter's parents have ulterior motives and are indeed bad people.
At school, Peter faces further bullying when Brian, a schoolmate, destroys his treasured pumpkin. Encouraged by Sarah's influence, Peter retaliates by pushing Brian down the stairs, resulting in the boy suffering a broken leg. Consequently, Peter is expelled from school, leading to a punishment inflicted by his parents—he is locked in the basement behind a refrigerator. Here, he uncovers a peculiar pit concealed beneath a grate.
Miss Devine later visits to check on Peter but is intimidated and chased away by Mark, who bears a suspicious gash on his arm, claiming it is from home renovations. Eventually released from the basement, Peter reconnects with Sarah, who reveals that she is actually his older sister, locked away by their parents who had deemed her unwanted. Sarah incites fear in Peter, convincing him that their parents plan to kill him. To save himself and escape from shared grim fate, Peter is led to believe that he must kill his parents, which he does by surreptitiously adding rat poison to their soup during dinner. As they attempt to intervene, both parents succumb to the poisoning, with Carol’s final words cautioning Peter to “not let her out.”
With the unfortunate deaths of his parents, Peter discovers a hidden door behind the grandfather clock in their bedroom, unlocking a pathway for Sarah to crawl out, though she appears almost unhuman in the dim light. In a shocking revelation, Sarah admits to Peter that she has been manipulating him into taking revenge on their parents for imprisoning her and holds resentment over Peter’s ability to live a normal life.
Soon after, Brian and his cousins arrive, intent on revenge against Peter. However, they meet a gruesome end at the hands of Sarah. Just then, Miss Devine arrives, drawn to the chaos by a phone call from Peter. Together, they manage to trap Sarah back in the basement pit. Even in defeat, Sarah chillingly warns Peter that she will one day escape and haunt him forever, leaving a lasting impression of terror and uncertainty in Peter’s young life.
