I've never heard of this movie before until someone mentioned it as good inspiration for a Carcosa campaign. And I was surprised how good it is. From a black and white low budget movie set in the English civil war and shot in 2 weeks, you don't usually expect a lot.
It's about an alchemists apprentice who is hunting for one of his masters partners/competitors, who has stolen some very important manuscripts. After passing through a hedgerow to escape from a battlefield, he meets three soldiers who are also trying to get away. Nobody said anything about deserting or cowardice, they are just going to get a drink at an alehouse that is supposed to be somewhere nearby. When they find a strange rope lying in the grass, things are starting to get really Weird. With capital W. The field is clearly not entirely off this world, and they and up finding a fifth man, who happens to be the rogue alchemist who stole the manuscripts. And as he is telling them, he came to this field to find a treasure, and they are going to help him. And things get increasingly downhill from there.
The movie could be called Horror, though it's not exactly scary, or even Mystery, but there is no real investigation and discovery of what's going on. Interstingly, almost all reviews and comments call it a movie about drugs and hallucinations, but the director himself said that that isn't at all how he is seeing it. To me, it looks like a pretty much straightforward movie about lovecraftian magic. The mushrooms that feature prominently in the film seem to be more like amplifiers of the supernatural, not the source of figments of the mind. It's about a dangerous supernatural place, not about some mystic journey of self reflection. Treating it as a movie about some down on their luck losers who get high would be selling the movie short and missing out on a lot of what seems to be going on.
I'm really not quite sure what to take from it for the presentation of magic in my games, but I know I really want to. There's no sparkles or fire or lightning, but a fair deal of things that should not be. Or rather, not be happening.