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"Be Warned: You will actually see a man turned inside-out."
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) and his beautiful captive Amanda Marvin (Barbara Bach).
Amanda's father, Professor Ernest Marvin (Joseph Cotten), a once-famed biologist, has discovered a way to transform humans into amphibious creatures and controls their every move. Rackham manipulates Marvin into performing the procedure upon both willing and unwilling participants by assuring him that his work is undertaken for purely scientific and humanitarian motives (Marvin hopes to reduce strain on the world food supply by creating a race of people who can live in the resource-untapped ocean). Having discovered the lost city of Atlantis beneath the waters surrounding the island however, Rackham is in actuality using the half-human monsters to plunder the lost city of its treasures.
Shakira (Beryl Cunningham), a voodoo priestess in the employ of Rackham foretells death and destruction descending upon the island.
The priestess' prophecy is fulfilled as the film ends with Claude and Amanda attempting an escape from a gun-wielding Rackham, a crazed Shakira, uncontrolled fishmen and the very volcano that doomed Atlantis which awakens and threatens to send what unsubmerged landmass remains to oblivion.