In this movie, jewel thieves and detectives hunt for diamonds that keep vanishing, and everyone thinks everyone else has them. As it turns out, the culprit is not human, but a giant jellyfish/octopus monster that lives in the sky and feeds on carbon (diamonds, coal, etc.).
This is one of the strangest monsters Toho ever produced; not a man in a monster suit, but a rarely seen half-puppet half-animated creation with tendrils that reach down out of the sky to retrieve what it wants. This is the only appearance of this monster; it did not even appear in DESTROY ALL MONSTERS, as it really doesn't work on the same level as Godzilla and his friends. In fact, so much time is spent on the gangsters, detectives and scientists that the monster almost feels like a subplot rather than the main subject of the movie. I believe the movie had a satiric and/or comic intent, but only traces of this can be found in the somewhat confusedly dubbed American version; I would very much like to see a subtitle version of this one day.