Hammer Studios fantasy horror. When Captain Lansen (Eric Portman)'s leaky cargo ship is blown off course, the crew members find themselves in a mysterious, fog-shrouded ocean where the seaweed eats human flesh and mutated crab-like creatures with huge claws are on the loose. The surviving crew and passengers then encounter a band of Spanish conquistadors who think it is still the 1500s and the Inquisition is still in progress.