Captain Joe Ryan is salvaging for treasure off the coast of Ireland when a volcano erupts, nearly sinking his ship. Ryan and his First Officer and friend, Sam Slade, take the ship to Nara Island for repairs. Before the Harbour Master, Mr McCartin, arrives to assist them, the crew meet Sean, an orphan, who assists McCartin: he invites them to see his collection of ancient Viking relics. Ryan finds himself intrigued by a relic bearing the image of a creature that Sean calls "Ogra, the sea spirit".
Joe and Sam consult the surly McCartin and discover he has more than archaeological interest when he shows them his stash of illegally salvaged gold relics from the harbour. Ryan warns him to stop being unhelpful or else he will call the police. As Ryan's divers check the harbour for other divers who mysteriously disappeared, one of them emerges and dies from "fear". That night a group of fishermen go out to check the water. They are attacked by a gigantic creature, but successfully drive it off with firebrands.
After meeting the fishermen and with help from McCartin, Ryan and his crew manage to capture the creature and haul it onto their ship. Sean warns them that they have made a mistake, but the crew ignore him. Hearing of the creature, two university scientists arrive to meet Ryan and Slade, hoping to obtain it for scientific study. However, Ryan has already sold the creature to Dorkin's Circus in London.
Dorkin names the creature "Gorgo", after the gorgon Medusa, before transporting it to a large, enclosed pit for public exhibition in Battersea Park. Ryan and Slade meet the scientists again, who inform them the creature is an infant. With its larger mother still out there, the Royal Navy takes charge of the operation. Later that night Gorgo's mother emerges from the sea and attacks Nara Island in search of her offspring before leaving. When she attacks again in the morning, a Royal Navy destroyer intercepts her, but she proves immune to their weapons and sinks it, killing all aboard.
The mother later attacks London. The military intervene, but fail to stop or destroy her. Ryan and Slade split up to find Sean, with the former finding the boy and keeping him safe. Gorgo's mother is eventually reunited with her offspring and frees him from the pit before they both return safely to the sea.
A salvage vessle is nearly sunk off the Irish coast by an undersea earthquake. A few nights later, a walking sea monster tangles with the fishing boats and enters the town. The salvage vessel captures Gorgo and takes it to London for display. Gorgo's mother, who is upset and significantly larger follows his trail to London leaving a wake of destruction in her path.
