In northern Italy, a clandestine gathering of Roman Catholic clerics, presided over by a shadowy robed figure, signals impending danger, as foreseen by Monsignor Franchino (Thibault de Montalembert). Simultaneously, in the bustling energy of New York City, Alison Parker (Cristina Raines), a top fashion model, feels the pressure of her boyfriend, lawyer Michael Lerman (Chris Sarandon), who is eager to marry her while she is in search of her own space.
Tragedy strikes when Alison's father dies unexpectedly in Baltimore, Maryland. This leads her to a painful recollection of the trauma she endured from discovering him in an orgy, a disturbing experience that once drove her to the brink of suicide. She finds solace in a cross that had been ripped from her neck during that distressing incident. Her mother, having endured her own struggles, reveals the reasons behind her choices, woven with desperation and survival.
After returning to New York, Alison interacts with an unusual real estate agent, Miss Logan (Ava Gardner), who presents her with an ideal Brooklyn Heights apartment. However, part of the charm includes sharing the building with a blind and senile priest, Father Halliran (John Carradine), who sits motionless by his window.
As Alison settles in, she experiences a frightening episode during a photo shoot which leads to a visit from her strange neighbor, Charles Chazen (Burgess Meredith). He brings along his pet cat and bird and shares tales of the other peculiar tenants in the building. As tensions rise, Michael pushes for marriage during a gathering at their apartment.
The following day introduces her to two more eccentric residents: the sarcastic Gerde (Sylvia Miles) and the mute Sandra (Beverly D'Angelo), who are involved in a seemingly affectionate relationship. Alison’s mental state begins to fray, as she collapses once more during a commercial shoot, much to the chagrin of her photographer, Jack (Jeff Goldblum).
Charles's absurd birthday celebration for his cat unveils more of her peculiar neighbors. That night, a disturbing dream pushes Alison to wake up amidst startling noises from the empty apartment above. After speaking to Miss Logan, she learns that the building, owned by the Holy Diocese of New York, has been largely uninhabited, save for the priest.
Meanwhile, Father Halliran has his own clandestine meetings with Monsignor Franchino. A private investigator, James Brenner (Hank Garrett), investigates Alison’s unsettling surroundings at Michael's behest. Alone one evening, Alison investigates the building further as she encounters hallucinations of her deceased father, leading to a violent episode.
Detectives Gatz (Eli Wallach) and Rizzo (Christopher Walken) suspect Michael's involvement in her chaotic experiences as they uncover eerie connections between the residents and a string of historical murders within the building. Her existence feels increasingly interwoven with a disturbing fate, as she confesses her darkest thoughts to Monsignor Franchino.
Upon further exploration, Michael realizes that Alison might be destined for a terrifying transformation similar to other tragic figures in the building’s history. As he unearths chilling revelations, it becomes evident that her future is intertwined with sinister forces aiming to manipulate her into darkness.
The plot thickens as Monsignor Franchino reveals the morbid truth lurking amongst the shadows. In a shocking twist, Michael, now revealed to be deceased, unveils that the building serves as a passage to Hell, with its residents being reincarnated devils intent on driving Alison to suicide in order to embrace her role as the next sentinel.
As chaos spirals, Alison confronts the demons embodying her deepest fears, including her own father and Charles. With her spirit tested, she must choose between succumbing to despair or wielding the cross that means salvation. As dawn breaks, the once-dreaded apartment transforms into a protection against evil, shrouding the dark past within its walls.
In the end, the cycle perpetuates as Alison, unsettlingly silent in her new role as Sister Theresa, now resides in the very apartment she once feared, keeping watch over the threshold to darkness, silently gazing from the window while holding the pivotal cross—a new sentinel guarding the gateway to Hell.
