The sunken Caribbean city of Port Royal had been long rumored to have been visited by pirates who rise from the ocean floor. In the mid-17th century, the port was a thriving seaport, but it was heavily damaged in 1692 by an earthquake and had suffered numerous hurricanes which had prevented the port from regaining its former glory. A 1907 earthquake caused the city to sink beneath the waves.
A scientist (Tony) believes that the older portions of the city will soon become visible due to predicted volcanic activity, and after obtaining grant funding, wants to investigate.
Before arriving on the island, thugs local to the area plan to steal all the gold when Port Royal becomes visible. They attack Tony, leaving him hospitalized and steal his research material in their quest to find the rumored pirate's gold.
Tony escapes the hospital, and arranges passage to the island. He is forced to take Joan with him, and she is disgruntled having been forced to accompany an exhibition whose theory she finds unlikely.
The ruins are now visible as predicted and the criminals obtain the treasure. The two groups happen to meet and are suddenly attacked by giant crabs. Volcanoes begin to erupt as the two groups fight for the treasure and to escape the now sinking city.