While on a routine mission, Cmdr. Pete Mathews (Kenneth Tobey) runs into trouble when his submarine is nearly sunk by an unknown creature. Back at base in Pearl Harbor, Dr. John Carter (Donald Curtis) and Professor Lesley Joyce (Faith Domergue) identify the beast as a giant octopus from the nether reaches of Mindanao Deep, which has been awakened by nearby nuclear testing. Radioactive and monstrously huge, the rampaging leviathan is heading toward the North American Pacific Coast.