Marc and his younger sister Arndis are spending some time in the woods of Nomen Tuum, where Marc can recover from his war injuries by drinking from the convenient fountain of youth thereabouts. Unfortunately for them there is a couple of succubi around who spend all their time corrupting men and they are hungry for the challenge the incorruptible Marc presents to them. Also, Arndis really enjoys staring at eclipses to the point of blindness, which only makes things more difficult for the siblings.
You can't talk about this movie without mentioning that it was filmed in the imaginary moon language of Esperanto. This is a language that was developed to be a sort of second universal language, though the fact that it sounds pretty much like various European languages all stuck together would have made things difficult for 20% of the world population, who speak primarily Chinese which has absolutely nothing to do with Esperanto. So why come up with a whole new manufactured language? Just use Chinese or something and write it with the roman alphabet so people don't have to learn those squiggly characters. This way you would start with 20% coverage! Esperanto dreams of 20% population coverage.
So Esperanto is a miserable failure of an idea, but how is Incubus? It's better in a lot of ways. For one, it has the only successful international manufactured language in it- that is, William Shatner. Whether it's because it is in the totally nonsensical fever dream language of Esperanto, or because Shatner hasn't developed his style fully, we can watch a toned down Shatner, who calmly and assuredly delivers honeyed words of poetry for us for the entire duration of the movie.
It's not a particularly interesting story. It's basically a medieval morality play on film, but the atmosphere is other-wordly and the goat that shows up at the end is really creepy. Goats are scary, what can I say. The cinematography is really outstanding as well, with nice shadows and an overall moody presentation. Unlike junky 50s scifi flicks, movies like Incubus really play to the strengths of black and white film making.
If you watch this movie with a bunch of pals you are going to be making Star Trek jokes all throughout at poor Mr. Shatner's expense so I would recommend instead to watch it on a rainy, gloomy morning when you have nothing much going on. Take a trip to Nomen Tuum and you might be surprised at what you find.