The film is allegedly based on the life of the real Carla Moran, who claims to continue to be assaulted by this invisible being, though with lesser frequency and intensity. It opened to major critical panning and was called misogynistic and unneedingly sexually graphic. Despite it's critical and commercial failure, it is widely considered to be a horror classic among the same lines as The Exorcist or Poltergeist.
In the film Carla lives with her children in a home in Los Angeles. One evening she is violently raped by an unseen being, and the molestations and attacks continue. She goes to a local college campus to see counseling from a therapist (played by Ron Silver) because she suspects she is suffering from some sort of mental illness. He concurs, suggesting that she is manifesting hallucinated experiences based on a fear of sex, possibly instilled in her by her unbringing.
However, after another rape, she seeks solace at the home of friends who witness paranormal phenomena. Finally assurred of the legitimacy of her experiences, Carla seeks help from a paranormal research team. They accompany her to her home to try and help, and it seems that they succeed. Unfortunately when a lover comes to stay the night he leaves the room briefly only to return and find Carla nude and pinned to the bed by an unseen force. Her breasts are being pushed in and manipulated and she is begging him for help.
Eventually the team of parapsycholoists come up with a plan. They set up a home-like location with guns prepared to spray everything with liquid nitrogen. They presume that since the being can manipulate physical things, it must have physical mass, and ergo can be frozen. Unfortunately the being does strike and is so powerful that it takes control of these spray nozzles and attempts to kill Carla with the nitrogen. Ultimately she faces off with it, (though she can't see it) stating that it can kill her, torture her, but it will never rape her again. In an act of fury the being bursts the liquid nitrogen tanks and freezes itself, revealing that it is absolutely gigantic in size. All this is witnessed by Carla's psychiatrist, who until now did not believe she was telling the truth.
Ultimately Carla returns home only to be confronted by the entity again, and she is simply forced to live with its presence and try to be unafraid.