Deep-sea divers Joe Ryan and partner Sam Slade encounter an extraordinary prehistoric creature off the coast of Ireland, near the remote island of Nara. When an underwater volcanic eruption damages their ship, they are forced to dock on the island for repairs. There, they witness the creature, christened "Gorgo" by the local islanders, who revere it as a legendary sea monster. Sensing opportunity, Ryan and Slade capture the enormous beast, intending to exploit it as a spectacle. Defying the warnings of the islanders and a young boy named Sean who has formed a unique bond with Gorgo, they transport the creature to London.

In London, Gorgo becomes the center of a media frenzy, drawing crowds from far and wide to marvel at the captured giant. Hosted in a circus-like environment, Gorgo is presented to the public, with the promoters reaping financial rewards as curiosity turns into a lucrative attraction. As the days pass, scientists studying Gorgo realize that it’s far from fully grown, suggesting a larger creature may exist. Meanwhile, Sean, deeply concerned for Gorgo's well-being, attempts to make his way to the city to rescue the creature he empathizes with, reflecting growing concerns about the ethics of exploiting such a magnificent being for entertainment.