After her estranged mother's death, Linda Stevens inherits a rural estate called Montclare, which has been in her family for years. The rambling structure houses a long-held retirement community for the elderly. Linda arrives on the same night as a new boarder, the aged Mrs. Ryan, dropped off by her son Kelvin. Shortly after Linda settles in, a resident of the home is found drowned in a bathtub.
Linda begins reading childhood diaries left in the house by her mother, in which she describes being unsettled in the home and believing someone is watching her. She also finds handwritten medical records for her aunt Rita, which disturbs her as she was told that Rita had been dead at the time they were written. Meanwhile, Linda reconnects with Barney, a man whom she was romantic with years prior. After the two go on a date together, Linda returns to Montclare and is notified that Lance, another resident, has suffered a debilitating stroke. Later, Linda informs Dr. Barton, the resident physician, that she is considering selling the property.
After a series of unusual incidents, Linda comes to suspect that Rita may still be alive and stalking the house, though Dr. Barton assures her that Rita died in a psychiatric hospital years prior. Upon further delving into her mother's diaries, Linda finds that there is a precedent for mysterious deaths in the retirement community, particularly an inordinate number of drownings. Convinced that Dr. Barton and another employee, Connie, are murdering residents, she flees to town on foot and locates Barney in a panic.
Barney escorts Linda back to Montclare and enters the house to obtain the diaries. Outside Linda finds Carol, a romantic rival previously introduced, with her throat slashed. Linda flees into the house in search of Barney, only to find him also murdered. Upstairs, she finds Lance, conscious and able to walk. Linda ushers him outside through a window onto an emergency escape stairway. She is soon confronted by Mrs. Ryan, who is in fact Rita, seeking vengeance against Linda's mother for placing her in the hospital; Rita claims that it was Linda's mother who was in fact insane. A crazed Kelvin—Rita's son (and Linda's cousin)—attacks Linda with a sledgehammer.
Linda manages to barricade herself in a bathroom, where she finds Dr. Barton's and Connie's bloodied corpses in the bathtub. She stabs Rita through the eye with the sharp handle of a comb as Rita peers through the door lock, then manages to escape by car. She arrives at the local diner nearby, where the owner's young son, Nico, is alone. The two barricade themselves in the diner, but Kelvin arrives, driving his truck through the side of the building. Using a shotgun, Linda manages to shoot and kill Kelvin. At dawn, the two drive away as the diner becomes engulfed in flames, ignited by the petrol leaking from Kelvin's damaged truck.
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