The film opens on three "students" (i.e., guys in their mid-30s) climbing a mountain. One of them loses his footing, and falls. A struggle ensues to pull him back up, and his friends discover that he no longer has a head.
Meanwhile on a passing train, a young woman faints. Her name is Anne, and she is traveling to Geneva with her sister Sarah. The two of them have a mind-reading act in London. Dr. Allen Brooks, a United Nations scientist, is in the same car, and comes to Anne's aid She relates that she has had some sort of psychic episode. She needs to get off the train at the next stop, Trollenberg.
Allen travels from Trollenberg to a well-armored1 observatory on the side of the mountain, where he learns that the the south side is covered in a radioactive cloud, similar to one in the Andes a few years ago. Both sites have seen inexplicable events.
There follow a series of decapitations, and some mind control, some exploration and a whole lot of alcohol consumption ("Welcome to Trollenberg! How 'bout a little nip before you unpack your bags?" "Now that we're acquainted, why don't you get settled into your rooms? I'll send up a bottle of wine. Come down when you're ready, and we'll have a little drink." "I need a post-drink drink--and after that maybe we can adjourn to the study for a brandy."). Anne's psychic powers come in handy, and Sarah is mostly there as set dressing. Eventually the Crawling Eye shows up, and it has tentacles.
The Trollenberg Terror is not as underwhelming as I make it sound. Not every night is a good night for me to see a movie, and this is one where I might normally have done something else. As it was, I spent a lot of time playing with my phone. I'm not sure how I'd have gotten through the movie in the days before smartphones, but I'm sure I'd have found some other way to ignore 30% or so of the film.
It is not a bad movie, however, at least not in the context of monster movies from the '50s, and it scores a couple of extra Weirdness Points for being British. I find mid-20th century British sci-fi to be interesting in an impenetrable way: they were operating on a totally different wavelength over there. If Star Trek and the original version of Battlestar Galactica were quintessentially American shows, then Doctor Who and Blake's 7 were their quintessentially British counterparts, and the differences are startling.
Here's the trailer.
1This is not the sort of detail you dwell on in a movie for no reason. Like Chekhov's Observatory, this one is fired in the third act.