In northern Italy, a group of Roman Catholic clerics led by a shadowy robed figure (Jose Ferrer) meet and Monsignor Franchino (Arthur Kennedy) portends danger.Meanwhile, in New York City, elite fashion model Alison Parker (Cristina Raines) seeks her own apartment, but her boyfriend, lawyer Michael Lerman (Chris Sarandon), wants to get married.In Baltimore, Maryland, Alison's father dies suddenly and she returns home, where she recalls being so traumatized from walking in on her father participating in an orgy that she attempted suicide. She collects a cross that her father tore from her neck during the incident. Alison's mother tells her she stayed with Mr. Parker because she had nowhere else to go.Back in New York City, Alison meets with real estate agent Miss Logan (Ava Gardner) who shows her a "perfect" apartment to rent. Alison rents a furnished apartment in Brooklyn Heights. On the top floor of the building lives a blind, senile priest named Father Halliran (John Carradine), who stares blankly out of his window.After fainting during a photo shoot, Alison is visited by an eccentric neighbor, Charles Chazen (Burgess Meredith), who drops by with his pet cat and bird and tells Alison about the other tenants in the building. When he leaves, she notices a photograph of Charles in her apartment. That night, at a party in the apartment, Michael again presses Alison for marriage.In the morning, Alison meets two more neighbors, the wisecracking Gerde (Sylvia Miles) and the mute Sandra (Beverly D'Angelo), who are lovers.At a commercial shoot, Alison again collapses, frustrating Jack (Jeff Goldblum) the photographer.Charles throws a birthday party for his cat and Alison meets more of her strange neighbors. Later, she has a disturbing dream and is awakened by noise from the supposedly empty apartment above her.Alison meets with Miss Logan and learns that aside from the priest, the apartment building has been empty for years and the building is owned by the Holy Diocese of New York. Meanwhile, Father Halliran is visited by Monsignor Franchino. (In some versions, Franchino meets with a group of priests who are a secret order to protecting the world from evil to give them an update them on the events unfolding.) Elsewhere, Michael Lerman asks private investigator James Brenner (Hank Garrett) to check out Alison's building.Alone the next night, Alison hears more noises and investigates. She sees Chazen's cat eating the pet bird and is pursued by an apparition of her father, whom she stabs repeatedly. Alison flees into the street, where she is found, covered in blood, by residents of nearby buildings. The next day, two police detectives, the veteran Gatz (Eli Wallach) and his rookie partner Rizzo (Christopher Walken), visit Michael and question him about his dead wife Karen's suicide and a second suicide attempt by Alison, implying that he is involved in the previous night's incident. Meanwhile, police find nothing suspicious in Alison's apartment and match the blood type on her nightgown to her own.Michael discovers that Mrs. Clark, one of the supposed guests at Chazen's cat's birthday party, was the name of a convicted murderer, who was executed in 1949. Later, Brenner, the private eye, is found stabbed to death in his car and his blood type matches Alison's. Detective Gatz connects Brenner to Michael and suspects that Michael hired the investigator to kill Karen and is now trying to drive her insane. When the detectives arrive at Michael's apartment to confront him, Alison leaves her boyfriend's home for a walk and stops by a Catholic Church. There, she confesses to Monsignor Franchino that she has committed adultery, twice attempted suicide, and is feeling suicidal again. The monsignor assures her that if she embraces Jesus and returns to the church, the feelings will vanish.When Alison returns to Michael's apartment, he suggests that she may not have imagined the events and they visit her apartment in Brooklyn Heights. There is no evidence that Alison killed anyone there, but she says that some of the things in the room, such as the carpet, are different. In a book, Alison sees the same Latin phrase on every page, but Michael only sees English. When Alison writes the text, Michael takes it to Professor Ruzinsky (Martin Balsam), who reports that it is a passage from Milton's Paradise Lost.Alison returns to the church and meets the rector, who tells her he does not know of the priest she claims to have met there earlier. Meanwhile, Michael visits Monsignor Franchino under the pretense that he has a bequest for Father Halliran. Franchino tells him that Halliran has been retired since 1952, suffers from palsy and is unable to accept visitors. Michael shows Franchino the excerpt from Milton, but the monsignor claims not to recognize it. Franchino refuses Michael's request to make a copy of Father Halliran's file. That night, Michael asks Alison's friend, Jennifer (Deborah Raffin), to stay with her while he attends to business.Michael meets a man named Perry (William Hickey), an ex-convict that he represented in the past and bribes him some cash to assist with breaking into the diocese to look at the official files on Halliran. Michael discovers that Father Halliran was originally named Daniel O'Rourke, and, after a suicide attempt, disappeared and re-emerged as Halliran. Michael finds files on several more people who attempted suicide, gained new identities and became priests or nuns and all have lived and died in the very same apartment building in Brooklyn where Alison lives. When one dies, another takes his or her place. A file for Alison states that Halliran will die the very next day and Alison will become a woman named Sister Theresa.Meanwhile, detectives Gatz and Rizzo determine that the eight people Alison claims were at the birthday party for the cat were murderers executed long ago.Michael has Alison attend a party at Jennifer's while he investigates the apartment in Brooklyn Heights. At the party, Alison faints and her friends put her to bed, but at midnight she sneaks away. In Brooklyn Heights, Michael discovers a hidden portal behind a wooden wall of the building which reads: "Through me you go into the city of grief. Through me you go into the world of pain. Through me you walk among people lost. Abandon hope all yee who enter here". Father Halliran appears and tells him that this whole building is the entrance to Hell. Michael pursues the priest upstairs and demands answers as he attempts to strangle him.Alison arrives at the building and is met by Michael, who calmly explains that the Latin they found was a warning from the angel Gabriel to the angel Uriel, stationed at the Gateway to Eden. Since that time, a series of sentinels have stood watch, guarding the world against evil. Currently, it is Father Halliran, but that night, Alison is to become the next sentinel upon his passing. The people Alison met in the building, the murderers, were actually reincarnated devils, who could only stop her from becoming the next sentinel by driving her to suicide. Michael goes on to explain that he is dead, killed by Monsignor Franchino minutes earlier for trying to strangle Halliran. He is damned to eternal hell for his sins, including hiring Brenner to murder his wife, Karen. Michael turns his head, revealing a gaping wound, and his head splits open.Alison flees downstairs where Chazen and the demons confront her. She returns upstairs and sees Gerde and Sandra feasting on Michael's dead body. Just then, all of the demons of hell come forth from behind doors and walls. Among the demons, Alison sees her father and Chazen. Cornered by the deformed and hideous demons in Halliran's apartment, Chazen approaches Alison and urges her to join them by killing herself; he gives her a dagger. Franchino and Halliran appear bearing a cross and drive back the demons. Chazen exhorts the demons to fight, but Alison takes the cross. The devils fade back into the walls and the monsignor leads Father Halliran away.The next morning, Gatz and Rizzo arrive at the apartment building where Father Halliran is carried out of the building in a body bag, with the medical examiner's report that he just died of a heart attack. A search of the building leads to no other people around. The building is later demolished.Some time later, Miss Logan, the real estate agent who is still working for the shadowy priesthood in protecting the new apartment building, shows a young couple (Tom Berenger and Nana Tucker) an apartment in the new building. They ask about the neighbors and are told that there are only two: one is a musician and the other is a reclusive blind nun who lives on the top floor. In that apartment, the blind Alison, now known as Sister Theresa, sits silently, wearing a white habit and staring blankly out the window, holding the cross... now the new sentinel and protector of the entrance to Hell.