A nuclear submarine on patrol maneuvers in the Pacific Ocean comes into contact with something the sonar determines is massive. The boat is disabled but manages to free itself and dock at Pearl Harbor. There it is discovered animal tissue of great proportions has jammed in its propellers. A man-and-woman team of marine biologists is called in, and they identify the tissue as part of a gigantic octopus. The military authorities scoff at this explanation, but are finally persuaded to investigate upon receiving reports of missing bathers, and ships pulled under the water by some living thing. The scientists conclude the octopus is from the Mindanao Deep and has been forced out of its natural habitat due to hydrogen bomb testing in the area. The testing has rendered the octopus radioactive, and this radioactivity drives off its natural food supply. The Navy tracks the beast, first to Astoria, Oregon, where it attacks people on shore, leaving sucker imprints in the sand, and then to San Francisco. As a jet-propelled torpedo (capable of being detonated by remote control) is developed which can penetrate the octopus's thick hide, and reach its brain to kill it, it increases its forays onto land, first attacking the Golden Gate Bridge and then coming ashore at the Embarcadero, where it kills panicking crowds and destroys buildings, including the Ferry Building. The sub commander and the scientists elect to take on the creature underwater. The torpedo penetrates its skin, but the monster captures the sub in its grip, trapping it and making it impossible to set off the torpedo without destroying the sub. Two divers go out and annoy the octopus, which thereupon releases the sub. The sub escapes and detonates the embedded torpedo from a safe distance. All survive, except the octopus, which is presumed to have been destroyed.
The submarine used in the opening is the USS Cubera (SS-347), she was the first of 24 post-war "GUPPY-II" - converted fleet attack submarines of the Balao class.