There’s some sort of nuclear reaction involving heavy water taking place on the bottom of the sea, producing a deadly ray of light which we never actually see.
Then there’s some sort of hideous sea creature (which, of course, looks like a guy in a rather loose suit, with a big mask – sorta like Barney with lots of teeth. It would have been acceptable just two or three years earlier, before The Creature from the Black Lagoon). This is either caused by the ray of light or mutated from turtles by a scientist who probably doesn’t qualify as “mad.” Just a touch obsessive.
And, of course, we’ve got the usual collection of one-from-column-A-two-from-column -b subplots: the beautiful spy girl; the scientist’s lovely daughter; the guy trying to murder the hero; the prying secretary nursing a secret grievance; and not one, but two separate Federal Investigations! In fact, the hero is not only a Fed, but is also a noted scientist as well! (No, he isn’t a rock star, too. That was Buckaroo Banzai).
If you still haven’t figured out what kind of film we’re dealing with here, one should note that it was released by AIP, and produced by the same Milner Brothers who brought you the infamous walking tree in From Hell It Came.
So strictly a bottom of the barrel snooze-fest, although I like the monster, as silly as he is – and the opening attack sequence is actually pretty good.