At night, a carnivorous, shape-shifting alien named Kator (Barry Stokes) lands in the woods of rural England. The vanguard of an invasion force, his mission is to evaluate the suitability of humans as a source of food for his species. Stumbling across Anderson and Sandy (Sandy Chinney), a couple having a tryst in their parked car, he kills both and assumes the appearance of Anderson. The next morning, he encounters Jessica-Ann (Gloria Annen) and Josephine (Sally Faulkner), a lesbian couple who live in a nearby manor house. Although Jessica owns the property, having inherited it from her late Canadian parents, the dominant of the pair is Jo, who is unusually possessive of Jessica and deeply suspicious of men. Simon, Jessica's boyfriend, has mysteriously disappeared. The women are vegetarians and live in seclusion with only a few chickens and a pet parrot, Wally, for company.Calling himself Anders, and feigning an injured leg, Kator is taken in by Jessica and Jo. His arrival immediately causes friction between the two. Bored of her monotonous existence, Jessica welcomes the stranger's arrival. Jo, however, openly resents his presence and suggests that the socially-awkward Anders is an escapee from a psychiatric hospital (which she is herself has done).Later, having returned to the spot where he killed Anderson and Sandy, Kator/Anders kills and partly devours two policemen who are examining the couple's abandoned car.Back at the house, Jessica finds a knife and bloodstained clothes in a spare bedroom; recognizing the latter as Simon's, she realizes that he was murdered by Jo.The next morning, Jo is furious to discover that all the chickens have been slaughtered. Blaming a local fox, she lays traps for the animal and goes after it with a rifle, assisted by Jessica and Kator. When the hunt fails, Kator tracks and kills the fox on his own and presents it to Jessica and Jo as a trophy. The trio celebrate with a champagne party for which Jo dresses Kator in drag. A subsequent game of hide-and-seek brings out more of the hunter in Kator.Later, Jo is disturbed to find the fox carcass stripped bare and realizes that the animal was not caught in a trap as she and Jessica thought. Jessica angrily rejects her warnings about Anders, interpreting Jo's fear as jealousy and revealing that she knows the truth about Simon.The next morning, Jo arms herself with her knife and stalks Kator as he hunts swans on a nearby river. Her attempt to eliminate him is thwarted when he starts to drown, alerting Jessica with his screams. Jessica and Jo rescue Kator and take him back to the house. While the two women clean themselves up, Kator kills and consumes the parrot Wally.Jessica tells Jo that she is no longer willing to be controlled and is leaving with Anders. Outraged, Jo knocks Jessica unconscious and runs into the woods to dig a grave for her. On waking, Jessica seduces Kator. As they start to have rough sex, Kator's predatory instincts are stirred, causing him to revert to his natural form and tear open Jessica's throat, killing her. Having returned to the house, Jo walks in on Kator eating Jessica's dead body in the bedroom and attempts to flee but falls into the open grave just as Kator catches up with her. Jo screams as the scene fades to black.Some time later, Kator leaves the house and calls his mother ship on an alien transceiver. Hungrily watching two girls walk along the river, he advises his superiors to dispatch more of his kind to Earth.