In the sea of Ireland, the boat "Triton" captures a marine monster, stirred out of sleep, perhaps, by a recent storm. The animal, named "Gorgo", is transported to London and exposed to the curiosity of people, first in a circus, then, from the moment it shows its aggressiveness, in a well protected pool near the River Thames. Gorgo, for his enormous size, is, in truth, only a "pup": Very soon, its gigantic mother climbs out of the river's current to free it. The marine monsters, finally re-united, head for the open sea, leaving behind them a semidestroyed London.

The film explains in an elementary way, but clearly, who are the bad and who are the good. On one side are sailor-profiteers, impresarios without scruples, and a crowd excited by morbid curiosity; on the other side, an orphaned child who disapproved of the capture of the animal, and, naturally, the mother and her son Gorgo. Between the two sides, in a separate position, is a group of scientists who rage against the shameful circus exhibition and demand to study the creature in the laboratory.

Eugene Lourie's premise are made in the mold of films by Inoshiro Honda, but the ending is slightly different, in that, on other occasions, the monster is usually sacrificed by civilization, and here it [the monster] leaves victorious and returns safe and sound to its natural surroundings. There is little importance that he has sunk half of Her Majesty's [British] Fleet, as well as Big Ben and other historical London tourist attractions: Gorgo is symbol of nature that deserves respect.