The Entity was a violent and invisible ghost or demon that shared similiar qualities to the medieval incubus as well as poltergeists - the creature is infamous for its repeated rape of the female protagonist of the movie and although highly controversial the events are supposedly based upon events that occured in reality (though much like the Amityvile Horror or other paranormal events truth and fiction often blur).
The movie begins as Carla Moran, a single mother, is violently raped in her home by an unseen force. Initially convinced the attack was a nightmare, a subsequent episode of poltergeist activity causes her to flee with her children to a friend’s home.
Evidently followed by the entity, Carla is nearly killed the following day when her car mysteriously loses control in traffic. Urged by her friend Cindy to see a psychiatrist, Carla meets with Dr. Sneiderman and tentatively agrees to undergo therapy. An attack occurs that leaves bruises and other injuries on Carla which she shows to Dr. Sneiderman who believes she has caused them to herself. We learn that Carla suffered a variety of traumas in her childhood and adolescence including sexual abuse, teenage pregnancy and the violent death of her first husband. Dr. Sneiderman believes her apparent paranormal experiences are delusions resulting from her past psychological trauma. He urges her to commit herself to a psychiatric hospital for observation. She refuses.
After Carla’s friend Cindy witnesses an attack, the two explore possible supernatural causes. While visiting a local bookstore, Carla happens to meet two parapsychologists who she convinces to visit her home. Initially skeptical, the scientists witness several paranormal events and agree to study the home. During their study Dr. Sneiderman arrives and confronts Carla, trying to convince her that the manifestation is in her mind, but she dismisses him. Reassured that her case is being taken seriously, Carla begins to relax. Carla’s boyfriend Jerry visits and she suffers a particularly disturbing attack which he witnesses. Hearing the commotion, Carla’s son enters the room and believes that that Jerry is harming her prompting him to attack Jerry. Later at the hospital, Jerry is so troubled by what he saw, he ends their relationship.
Desperate for an solution to her problem Carla agrees to participate in a complicated study carried out by parapsychologists at a local university. A full mock-up of her home is created as a trap to lure the entity. Once inside, it would be frozen in liquid helium. Before the experiment can begin, Dr. Sneiderman arrives and unsuccessfully tries to convince Carla to leave. The entity eventually manifests as a cold wind and unexpectedly takes control of the helium tanks, attempting to kill Carla who defiantly stands up to it, stating that it can never have her. Dr. Sneiderman rushes in and saves her. As they escape the laboratory, they see the entity frozen for a brief period into a very large mass of ice. It eventually breaks free and vanishes. Carla returns to her house the next day. The front door slams by itself and a demonic voice greets her. She calmly opens the door, exits the house, and gets in a car with her family and leaves.
The film ends with captions stating that Carla's attacks have decreased in both frequency and intensity, but that she still experiences paranormal activities.
The Entity may be spirit of Carla's first husband or a demon (like many other nightmare spirits it may also be linked to the phenomena of sleep paralysis (though obviously given a supernatural flair for the film) ).