What we got here is your basic Japanese giant monster movie. Nothing too special. It begins like so many others have, with some Nazis retrieving Frankenstein's still-beating heart from a mad scientist's castle in Frankfurt and transporting it via submarine to a lab in Hiroshima, where the A-bomb causes it to regenerate into a full-grown monster that spends the next 15 years eating bunny rabbits in the radioactive ruins. Like I said, pretty standard stuff.
I don't want to belabor this review, because if you've seen one giant Frankenstein movie, you've seen 'em all. Some scientists working at a hospital for victims of the Bomb find Frank wandering around, so they bring him back to the lab for study. They helpfully explain that he's a Caucasian, even though he's very clearly played by a lanky Japanese dude. Then they discover that Frank will regenerate any damaged tissue as long as he gets a steady dose of protein. Then, for reasons not fully explained, he starts growing and growing and growing until he's about 50 feet tall. He breaks out of the lab and lives in the mountains, occasionally getting into some mischief, like when he throws a whole tree at a bird and it accidentally crushes some poor dude's log cabin. Oh, Frankie. You lovable scamp.
The problem is, there's this other monster, a real prick named Baragon who looks like a reptilian Labrador puppy with a glowing horn on his snout. He's tunneling all over the countryside, blowing up oil refineries and making Frank take the blame for it. So now the army's trying to kill poor Frank while the Hiroshima scientists are trying to save him. Then him and Baragon finally meet up and have a knock-down, drag-out fight across the Japanese wilderness. It's kind of funny, because Frank is just some dude with a big forehead, so it's extra ridiculous when he jumps on the rubber puppy-lizard's back and starts whaling away. At first, you don't think he has a chance because this Baragon asshole is so much bigger, and he can shoot red lasers out of his mouth, while Frank's just a skinny Japanese guy with no special powers. But that's actually Frank's secret weapon. Since he's not covered in 200 pounds of rubber, he has the mobility and speed to turn the tables on Baragon. Then Frank holds his corpse over his head victoriously and throws him over a cliff into the sea.
Any other movie would end right here, but not Frankenstein Conquers The World. It's got its own agenda. With only four minutes left in the movie, a random giant octopus (you can tell it's a giant octopus because one of the scientists yells, "Look! That's a giant octopus!") climbs out of the ocean to fight Frank. Maybe he was pissed off about Frank chucking that douchebag Baragon into his ocean, or maybe the octopus was really the mastermind behind the whole thing and Baragon was only his henchman. I don't know. The movie wasn't too specific about the octopus' motivations. Anyway, whatever his fuckin' problem is, this octopus is an even bigger prick than the last monster. Frank is already worn out from his battle with Baragon, so he doesn't put up much of a fight when Cocktopus drags him over the cliff to a watery grave. What a fuckin' bummer. Frank goes to all this trouble to clear his good name, and for what? Dammit, I hate that octopus.
Anyway, if you've ever wanted to see a movie where a random giant octopus is introduced in the last four minutes, this (and maybe the deleted scenes from Goonies) is probably your best bet. Glad I could help.