During the Cuban missile crisis, a Soviet submarine is on route to Cuba to deliver nuclear material to Castro. The sub is intercepted and torpedoed down by the US. Somehow this does not lead to nuclear war. The nuclear material sinks with the sub. I don’t know what exactly that nuclear material was and why Castro wanted it, let alone drums of it. There is no reason for him to want nuclear waste and enriched uranium is in rod form and plutonium is a metal. Maybe it was the ooze that mutated those turtles back in the day.
It does mutate, of course, but the world won’t know for 38 years.
Flash forward those 38 years and we meet a CIA agent. He’s an idiot. On his way from the CIA office in Bulgaria, he bumps into an old lady who is obviously and man in disguise. The disguised man gives a child some bread. The bread is a bomb. The CIA agent bumps into him again but doesn’t figure things out until the office blows up. Then he catches the guy who turns out to be a terrorist leader. They need him back in the USA and the best way to do it is… by submarine.
What I dislike about many animal attack films is how the writers couldn’t figure out a main plot that revolves around the animal attacks. The predominant story becomes something generic as hell, most often action related, while the animal attack is subjugated to a few scenes. If I watch an animal attack film, I don’t want B-movie grade Speed/Crimson Tide/Tom Clancy/James Bon/etc. I want animal attacks.
For the most part, you could totally remove the octopus (which was mutated into a really big octopus by aforementioned mystery ooze), and the movie would be the same. Its only real purpose is to attack the submarine at the exact times the CIA agent is about the capture the terrorist (he gets loose on the submarine, obviously).
Since the octopus is only in a handful of scenes, the mediocre actors have to carry the film while delivering inane lines from a forgettable script—the below par CGI isn’t helping them—which makes Octopus a forgettable yarn.