Nyah, a female commander from Mars, Nyah heads for London in her flying saucer as part of an advance Martian team seeking Earthmen to replenish the dwindling male population after a devastating war between the sexes. The entry rakes the craft with damage, and a near-miss with an airliner forces a crash-landing on the remote Scottish moors. Armed with a raygun that can paralyze or kill and accompanied by the tall, menacing robot Chani, Nyah begins her mission on Earth with a chilling resolve.
Astrophysicist Professor Arnold Hennessy, joined by journalist Michael Carter, is dispatched by the British government to assess the incident and its wider implications. Their investigation brings them to the Bonnie Charlie, a remote inn in the Scottish Highlands run by Mr. Jamieson and Mrs. Jamieson, where the locals and travelers converge and the tension between science and threat quietly builds.
Inside the inn, they encounter Ellen Prestwick, a fashion model who came to the Bonnie Charlie to escape an affair, and she soon forms a romantic connection with Carter. Meanwhile, a former convict, Robert Justin (operating under the alias Albert Simpson), arrives hoping to reunite with barmaid Doris, with whom he is secretly in love. The atmosphere thickens as the Martian presence lingers just beyond the door, and the inn’s ordinary life begins to fracture under the weight of impending doom.
Nyah makes her move, incinerating the Jamiesons’ handyman and entering the bar. When she finds no one willing to accompany her to Mars, she traps the guests and staff inside an invisible barrier and lets Chani loose to vaporize much of the manor’s grounds. Discovering Tommy and Robert Justin hiding on the grounds, Nyah abducts Tommy as a potential male specimen and mind-controls Justin, sending him back to the inn under her control. The tension in the room tightens as fear and fascination collide with the human impulse to resist.
Nyah then brings Professor Arnold Hennessy aboard her spaceship to view Martian technology, including the ship’s atomic power source. In exchange for Tommy’s safety, Carter volunteers to go to Mars with Nyah, setting in motion a risk-filled gambit to save Earth from a future of alien rule. Michael Carter makes his choice clear, while Hennessy hints at a plan to thwart Nyah from within.
Realising that victory may require deception, Hennessy suggests Carter sabotage the ship’s power source after take-off. But Carter’s plan collides with Nyah’s psychic control, and the attempted double-cross is foiled. Carter is released by Nyah and returns to the bar, where she declares she will destroy the inn and kill everyone inside when she leaves for London. Yet she leaves the door open for one man to accompany her to escape death, and the men draw lots to decide who will go.
In a dramatic turn, Robert Justin, now free from mind control and still at the bar, offers to join Nyah of his own free will. After take-off, he secretly sabotages Nyah’s flying saucer, sacrificing himself to save Earth and atone for the death of his wife. The few remaining survivors toast their narrow escape and resilience with a final drink at the Bonnie Charlie, the inn once again alive with life and memory.
