It is the year 1891 and a military doctor, Lieutenant Claude de Ross (Claudio Cassinelli) a survivor of not one, but ''two'' shipwrecks, washes ashore on a mysterious, uncharted Caribbean island along with a handful of convicts. When several of these convicts meet unfortunate ends at the hands of the titular fishmen, Claude and the other survivors flee into the jungle, only to encounter the sadistic Edmond Rackham (Richard Johnson (actor)) and his beautiful captive Amanda Marvin (Barbara Bach).