On the Swiss mountain Trollenberg, one of three climbers is suddenly killed, his head ripped from his body. Two sisters, Anne and Sarah Pilgrim, 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, in the same train compartment as the sisters, is going to Trollenberg's observatory to meet with Professor Crevett. Crevett goes on to explain that, despite a series of climbing accidents, no bodies are ever found, and an always-stationary radioactive cloud is regularly observed on the mountain's south face. Brooks mentions similar incidents that took place in
South America in the
Andes three years earlier, just before a similar radioactive cloud vanished without a trace. Local rumors circulated that something was alive in the heavy mist of the cloud. Anne is giving a
mind-reading demonstration at the nearby hotel when she
"sees" two men in a base camp hut on the mountain. Dewhurst is asleep when the other man, Brett, under some kind of mental compulsion, walks outside as the cloud envelopes the hut. Anne suddenly faints again. Brooks phones the hut, Dewhurst answers, screams, and then the connection suddenly goes dead. A rescue party, including Brooks, climbs up to the camp hut looking for both men. Anne, in a
trance-like state, urges the rescuers to stay away. Inside the hut, the rescuers discover that everything is frozen solid, despite the door being locked from the inside. Dewhurst's body is found under a bed with its head missing. A
spotter plane arrives and circles overhead, and a man is spotted a half mile away. The first rescuer finds a
rucksack at that location with a severed head inside. He is set upon and killed by Brett, who also kills the second rescuer when he arrives. Later at the hotel, Brett suddenly staggers in, claiming that he had been lost on the mountain. Soon after, he attacks Anne with a knife, but Brooks manages to subdue him. Brett sustains a severe head gash during the struggle, but no blood flows from the wound. He is heavily sedated and locked away. Brooks recalls to journalist Philip Truscott a similar incident in the Andes that followed a similar pattern: a man murdered an elderly woman who allegedly had
psychic abilities 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 woman. Brett escapes his improvised cell and resumes his hunt for Anne, this time armed with a handaxe. Before he can reach her, Brooks dispatches him with a pistol. Brett's flesh appears crystallized upon inspection and rapidly decomposes in the heat. The cloud has begun to move down the mountainside towards the hotel, so the guests retreat up to the fortified observatory. A mother realizes that her young daughter is missing as they enter the cable car. In a thickening mist, a giant tentacled alien 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. They return to the cable car, but the delay gives the 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 single cloud has now split into five while converging on the observatory. Hans, who left the hotel by car, suddenly turns up at the observatory. Once inside, he begins exhibiting the same obsession with Anne. Hans tries to strangle her, but Brooks and Truscott stop him as Brooks stabs him. As the aliens near the observatory, everyone makes
Molotov cocktails to combat them. By radio, Alan orders an aerial
firebombing raid against the observatory, which has reinforced walls and concrete roof that can withstand the assault. Truscott strikes one of the creatures with a Molotov cocktail, setting it ablaze. He is caught by a tentacle from another monster now atop the observatory's roof. Brooks sets that one ablaze with another Molotov cocktail, forcing it to drop Truscott. Later, Truscott firebombs another creature that manages to breach a portion of thick wall to get at Anne. The aerial firebombing assault begins and successfully torches the remaining aliens. ==Cast==