Abigail Clayton (Selma Blair) lives alone. Very alone. In fact, the heiress has not left her Manhattan apartment for almost two decades. When the death of her elderly neighbor prompts NYPD Homicide Detective Frank Giardello (Giovanni Ribisi) to launch an investigation, the agoraphobic Abigail is distressed to find him outside her door, questioning her. Having tried to acquire the now vacant apartment to ensure her privacy, Abigail is further upset when new tenants Lillian (Amy Smart) and Charlie (Jason Lee) 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.