Desperate to escape her criminal record and a life of living out of her car, Millie Calloway accepts a job as a live-in housemaid for the wealthy, influential Winchester family. She spends her days cleaning their gorgeous mansion and cooking meals for Nina Winchester, a highly unstable, erratic woman who seems intent on making Millie's life an absolute living hell. Millie is also drawn to Nina's handsome, deeply stressed husband, Andrew. However, as Millie moves into her tiny, claustrophobic room in the attic that locks only from the outside, she uncovers a dark, terrifying secret within the household walls that turns her saving grace into a battle for survival.