Driven from deep in the Pacific Ocean by A-bomb tests, a giant octopus attacks a nuclear submarine commanded by naval officer Pete Matthews (Kenneth Tobey) before turning towards San Francisco. With its radioactivity making it impossible to defeat with any conventional weapons, the beast wreaks havoc on the city, destroying nearly everything in its path, including the famed Golden Gate Bridge. Aided by marine biologists Lesley Joyce (Faith Domergue) and John Carter (Donald Curtis), Matthews struggles to stop the monster before it levels the entire city. And as a love triangle that is jeopardizing their mission, the trio mounts a last-ditch nuclear attack that brings the beast's rampage of death and destruction to an end.