At the remote Alpine village of Trollenberg, someone or something is killing off mountain climbers. The culprits turn out to be an army of one-eyed, tentacled beasts from outer space, who hide themselves in a radioactive cloud. Crawling along like snails, these horrific creatures attempt to neutralize their human foes by pumping a freezing fog at their pursuers. It matters not at all that some of the locals have been mentally enslaved by the monsters. It's up to American scientist Alan Brooks to save the day.
Tropes used in the film:
- Adult Fear: The woman who notices her little daughter is missing during the evacuation.
- Big Sister Instinct: Anne's sister wants nothing more than to get her the heck out of there.
- Brainwashed and Crazy: A couple of the mountaineers are hypnotized to try to kill Anne.
- Convection Schmonvection: The aliens attacking the observatory are defeated by having an airplane firebomb the observatory. But despite the fact that the aliens had already broken through the walls of the observatory in an attempt to get to Anne, the fires do not spread into the observatory itself and harm any of the people taking shelter there.
- Fainting: Anne does this a lot. Presumably it's due to outside influence.
- Gorn: This film is shockingly gore-soaked for the period in which it was made.
- Hostile Terraforming: The film is about aliens' native air slowly enveloping a mountain and later the surrounding area.
- Molotov Cocktail: Used to fight the monsters.
- Off with His Head!: The aliens' preferred method of murder, apparently for no other reason than the Body Horror.
- Psychic Powers: Anne has some level of mind-reading which she uses for parlour tricks. But she also seems to have a form of clairvoyance which draws her to Trollenberg and which let's her see what the aliens are up to (although she can't detect the aliens themselves).
- Spotting the Thread: The first victim is described as having been decapitated by his belaying rope when it tangled around his neck. It's quickly pointed out that the rope was secured about his waist and could not have gotten around his neck in the first place.
- The Stoic: Averted; everyone, including the men, react with realistic horror upon seeing the mutilated victims.
- Whole Plot Reference: Freakazoid! used this as the basis of an entire episode ("The Cloud").