First of all get a better title. 20,000 fathoms is nearly 23 miles. I’m sorry but the ocean just isn’t that deep. Even if the beast had been buried beneath the bottom of the ocean, 23 miles is a long way down.
The story starts out innocently enough, with a nuclear detonation at the north pole. The blast awakens a sleeping monster, who somewhat resembles Godzilla, only this beast walks on all four legs and doesn’t shoot a death ray out of its mouth.
The beast terrorizes ships in the North Atlantic. Of course no one believes these drunken old sea captains when they say that their ship was attacked by a thing that looks llke a dinosaur.
Finally the monster gets hurt and starts bleeding. Seizing the day, an Army sharpshooter shoots some sort of atomic bullet into the would, killing the monster.
The beast eventually comes ashore. But this time, instead of crawiling out of Tokyo Bay, he crawls out of New York Harbor and terrorizes the Big Apple by knocking down buildings, stepping on cars, and eating people.