Abigail Clayton lives alone. Very alone. In fact, the attractive heiress has not left her Manhattan penthouse apartment for almost two decades. The famous daughter of a wealthy industrialist, Abigail disappeared from the prying eyes of the press and the intrusiveness of her family on her 18th birthday, the day she received her massive inheritance. During years of self-imposed isolation, Abigail has had contact with only two people-her building's Concierge, Klandermann, with whom she communicates via notes-and Dr. Raymond Fontaine, a longtime family friend and her sole confidant for most of her life. When the death of her elderly neighbor prompts NYPD Homicide Detective Frank Giardello to launch an investigation, the agoraphobic Abigail is distressed to find him outside her door, asking to question her. Having tried to acquire the dead woman's now vacant apartment to ensure her privacy, Abigail is further upset when her requests go unanswered, and new tenants Lillian and Charlie move in. Abigail anxiously monitors her new neighbors from the safety of her front door's peephole, but her well-ordered world begins to unravel when she finally encounters Lillian and Charlie face-to-face-and her sheltered existence becomes threatened in unexpected and terrifying ways.