Description
On the Swiss mountain Trollenberg, one of three student climbers is suddenly killed, his head ripped from his body. Two sisters, Anne (Munro) and Sarah Pilgrim (Jayne), a London mind-reading act, are travelling by train to Geneva when Anne faints as the train passes the mountain. Upon waking, Anne insists that they must get off at the next stop.
UN troubleshooter Alan Brooks (Tucker), in the same train compartment as the sisters, goes to Trollenberg's observatory, where Professor Crevett (Warren Mitchell) explains that, despite many climbing accidents, no bodies are ever found; an always-stationary radioactive cloud is regularly observed on the mountain's south face. Brooks observes that similar incidents took place in the Andes three years earlier, before a similar radioactive cloud vanished without a trace. Local rumours circulated that something was living in the mist.
Anne is giving a mind-reading demonstration at the hotel when she "sees" two men in a base camp hut on the mountain: Dewhurst (Stuart Saunders) is asleep when the other man, Brett (Andrew Faulds), under some kind of mental compulsion, walks outside. Meanwhile, the cloud has enveloped the hut. Anne suddenly faints again, and Brooks phones the hut but no one answers.
A rescue party ventures to the hut looking for both men. Anne, in a trance-like state, urges the rescuers to stay away. Inside the hut, the group discovers that everything is frozen solid, despite the hut being locked from the inside. Dewhurst's body is found under the bed, its head missing. A spotter plane arrives and circles overhead, and a man is seen off in the distance. At his location, the first rescuer there finds a rucksack with a severed head inside. He is suddenly set upon and killed by Brett, who also dispatches the second rescuer.
At the hotel, Brett wanders in, claiming he had been lost. Thereafter, he launches a knife attack on Anne, but the men manage to subdue him. During the struggle, Brett sustains a severe head gash, but no blood flows from the wound. Brett is heavily sedated and locked away. Brooks recalls a similar incident in the Andes that followed a similar pattern: a man murdered an elderly woman who allegedly possessed psychic abilities just like those displayed by Anne. The killer's body was discovered to have been dead for at least 24 hours prior to his murder of the old women. Brett escapes his improvised cell and resumes his hunt for Anne, this time armed with a hand axe. Before he can reach her, Brooks dispatches Brett with a pistol. Upon inspection, Brett's flesh appears to be frozen, and rapidly melts into nothing in the heat.
The cloud has begun to move down the mountainside towards the hotel, so the group retreats to the heavily fortified observatory. As they enter the cable car, a mother realises that her young daughter is missing. In a thickening mist, a giant, multi-tentacled creature with a single huge eye appears at the hotel, smashing down the front door. Brooks manages to rescue the child from the lobby, both of them narrowly escaping its tentacles. They return to the cable car, but the delay has given the thickening mist a chance to reach the car platform. The transport motor begins to freeze, starting and stopping, the cable slipping, but the cable car arrives safely. The sole cloud has now split and become five while converging on the observatory.
Hans (Colin Douglas), who left the hotel by car to get help, suddenly turns up at the observatory. Once inside, he begins exhibiting the same obsession with Anne. Hans tries to strangle her, but Brooks stops him by stabbing him. As the large tentacled monsters near the observatory, everyone makes Molotov cocktails to combat them. By radio, Alan orders an aerial firebombing raid against the observatory, which has a reinforced concrete roof and walls that can withstand the assault.
Journalist Philip Truscott (Payne) strikes one of the huge one-eyed creatures with a Molotov cocktail, setting it ablaze. He is caught by a tentacle from another monster now atop the observatory's roof. With another Molotov cocktail, Brooks sets that one ablaze, forcing it to drop Truscott. Later, Truscott does the same to another creature that has managed to breach a thick wall in order to get at Anne. The aerial firebombing assault begins and is successful at torching all the remaining monsters.
A remote mountain resort in Switzerland is invaded by horrible alien creatures that like to decapitate humans. The beings are also in telepathic communication with people and inhabit a mysterious, radioactive cloud at the base of the Trollenberg mountain.
