It is a period piece conceived as being the movie that might have been made if Lovecraft had signed a movie deal in 1927 when the story was written; a silent film, in black and white, with cardboard backdrops and a stop-motion Cthulhu.
In the year 1927, a man now interned in a mental institution recounts his efforts in researching a network of cults based around the worship of an ancient, dormant god whose dreams inspire madness, insanity, and mass murder around the globe.
This film contains examples of:
- Alien Geometries: In the story, R'lyeh is said to defy any known dimensions, to the point where the sailors can't tell if a door is supposed to be a conventional door that opens horizontally, or a trapdoor that opens vertically, and a sailor is killed by Cthulhu when he fails to get past a corner which appears to be acute but acts as if it was obtuse. This is downplayed in the movie due to the low-budget 1920s style, though strongly alluded to with frequent shots of strange angles, and there is a scene wherein a victim falls into a crevice which an optical illusion has led the audience to believe is a convex crag of rock.
- Apocalypse Cult: The cultists apparently want to raise Cthulhu from the depths.
- Death by Adaptation:
- Inspector Legrasse survives in the story, but in the film it is mentioned at the end that he has also met his death.
- Captain Collins did die in the book, but he was killed in the battle for Alert. In the movie the crewmen simply find the derelict ship abandoned at sea, meaning that Collins instead dies by the hand of Cthulhu himself.
- Deliberately Monochrome: Invoked in the film in keeping with the style of 1920s cinema.
- The Determinator: Inspector Legrasse is so impressed with the cult's monstrosity that he invests a lot of time, energy and money to discover what lies behind it. Lampshaded when he says to the archaeologists the investigation turned into his own personal crusade.
- Downer Ending: The narrator finally understands what is really going on, but he also realizes that both the cult and Cthulhu himself are still alive, and realizes to his horror that he may die very soon. He winds up institutionalized, and the implication that his psychiatrist will soon follow the same path.
- Eldritch Abomination: Cthulhu himself. A, if not the, Trope Codifier.
- Eldritch Location: R'lyeh.
- Go Mad from the Revelation:
- Gustaf Johansen, following his experiences on the island.
- Thurston ends up in a mental institution after putting the story together.
- Island of Mystery: The island that pops up with R'lyeh on it.
- Octopoid Aliens: Cthulhu has a face that resembles an octopus, albeit crossed with a human skull.
- Religion of Evil: The Cthulhu Cult is built up as such. One of a swamp family even mentions that they normally don't want to associate themselves with the police, but are willing to do so at this point because of them.
- Retraux: The film is designed to look as if it might have been made shortly after the story was originally published in the 1920s.
- Sealed Evil in a Can: Cthulhu.
- Spared By Adaptation: The short story is presented as a document found after its author's death. In the film, he is still alive and telling the story in person. Though the final scene implies that he won't last much longer.