Entrepreneurs capture a giant sea monster not realizing it's really an infant with a protective mother.

Following an undersea volcanic explosion off the coast of Ireland, a gigantic 65-foot monster is released from its underwater home. After terrorizing the mainland, the beast is captured in a steel net by two salvage boatmen, Joe Ryan and Sam Slade, who decide to ignore the wishes of Irish paleontologists from the University of Dublin and take the creature to London for exhibition in the Battersea Funfair. They are accompanied by a young orphan boy, Sean, whom they have befriended. Though the little boy takes pity on the curious beast, now named Gorgo, he is prevented by Joe from setting it free. Word is received that Gorgo is merely an infant and his 250-foot tall mother is heading toward London in search of her offspring. Destroyers, flamethrowers, missiles, and jet airplanes all fail to halt the monster's progress, and the terrified London populace takes flight. Big Ben, Westminster Abbey, the Houses of Parliament, the Thames' huge Tower Bridge, and Piccadilly Circus are left in ruins by the mammoth beast's rampage. Only when it reaches its infant's side does the havoc subside. Peace returns as the two creatures make their way back to the depths of the sea.