Opportunistic English salvage divers Joe Ryan and Sam Slade are searching for treasure from a ship near the small island of Nara, which is off the west coast of Ireland. After an underwater eruption in which their ship is damaged, they go ashore where they meet McCartin, a secretive Government employee who discourages them from staying. Their interest in the area is reinforced when one of their divers surfaces with gold coins in his hand. However, they become alarmed when he dies from what seems to be fright. After further undersea disturbances a 65-foot-tall monster emerges from the sea and unleashes its wrath.