Although The Beast From 20,000 Fathoms is technically based on the Ray Bradbury story of the same name, it bares almost no resemblance to the story.* The action of the short story itself makes up less than five minutes of the film, and the rest is Hollywood. That’s no reason to dislike The Beast From 20,000 Fathoms though (unless you’re Ray Bradbury).
The Beast from 20,000 Fathoms is basically’ America’s Godzilla. According to Imdb, Godzilla (or Gojira as it was originally called) was released one year afterwards, and there’s some pretty obvious similarities. For one, both feature big dinosaurs rising from the sea, and in both the dinosaur is foiled by power lines. While Godzilla’s a hulking man-like lizard, the monster here is a stop-motion animated dinosaur made by the sfx genius and cult favorite Ray Harryhausen.
More often than not with monster movies there’s a few minutes of cool stuff but then a lot of filler. Even the most revered monster movies like Them (giant ant movie) have lots of filler. This isn’t an exception, but it’s brisker than most, and the actors are all highly animated so it’s rarely tedious to watch dramatic scenes while you wait for the dino to reappear.
The modern viewer will likely figure out many of the special effects right away. For instance, a lot of scenes feature the stop mation monster in the foreground and then have film projections in the background (similar to blue-screen effects today). It’s also a B-Movie that makes use of a lot of stock footage, even when it barely synchs up with the movie itself (like Ed Wood movies or The Day the Earth Stood Still). For me, knowing this didn’t ruin anything. I laughed a lot to think of the film makers’ ingenuity. I love things that are made by hand, and a stop motion monster is always better than a CGI one.
*Ray Bradbury’s short story was originally titled The Beast From 20,000 Fathoms but he later retitled it “The Fog Horn” to dissociate it from the film.