Nyah, a female alien commander from Mars, dressed in black vinyl, is heading for London. She is part of the advanced alien team that is looking for Earthmen to replace the dying male population on her world. Because of damage to her flying saucer, caused by entering Earth's atmosphere, she is forced to land her ship in the remote Scottish countryside. She is armed with a ray pistol that can paralyze or kill, and she also is assisted by a tall, menacing robot.
On Nyah's world, the emancipation of the women eventually led to open warfare between the sexes. The females won, usurping the political power of the men. This eventually lead to the sexual impotence of the planet's entire male population; a rapid decline in the birthrate soon followed. The aliens possess an organic, self-regenerating technology, which was used to construct Nyah's spacecraft. Against this technology, human weaponry proves ineffectual, as demonstrated when Nyah comes away unscathed by gunshots from a pistol. The alien technology is unreliable, however, and Nyah's people have not been able to use it to artificially produce new offspring.
The film is set mostly in the lounge bar of a Scottish country inn, "The Bonnie Charlie," somewhere in Inverness-shire. N'yah occasionally enters, makes threats, then leaves so the residents can contemplate her words. Intermixed with the Nyah story are a pair of romantic sub-plots that unfold in the film. In the first a fashion model, Miss Prestwick, fled to this remote country inn in order to escape a married reporter, Michael Carter, with whom she had an affair. Michael doggedly follows her, hoping to rekindle their romance. Meanwhile, a convict, Robert Justin, alias Albert Simpson , who accidentally killed his wife, has managed to escape from a prison in Stirlingshire and has come to the inn, hoping to connect with the barmaid, Doris whom he truly loves.
In the end, the earthlings manage to trick her, and place a bomb inside her spaship just before she flies off.