Somewhere in the middle of the Caribbean Sea, a group of men from a crashed prison ship, lead by Lt Ross (Italian regular Claudio Casselini) find themselves washed ashore on a remote island.
Wandering off in search of food and shelter, they predictably start getting picked off by some unseen creatures, which aren't typical of the local wildlife. After wandering round some more, the remaining survivors are surprised to discover the island is inhabited by an English man named Edmund Rackham (Played by British actor Richard Johnson).
Rackham initially offers them help and shelter, claiming that he and his colleague Professor Marvin (Joseph Cotton), are there to study the bizarre aquatic creatures who have been attacking them. But it actually turns out that the creatures are part of some genetic experiments the Professor has been conducting on the natives, as they need the Fishmen to recover a stash of lost treasure from the ocean floor.
But when the Fishmen finally decide to turn on Rackham and his henchmen, a furious battle erupts and Lt Ross finds himself in a race against time to try and get off the island before he too ends up as fish-bait.
Directed by cult Italiam Director Sergio Martino and co-starring Barbara Bach (Mrs Ringo Starr herself, who was another regular in Martino's films), whilst this may not be an original film by any stretch of the imagination, being heavily derivative of "Island of Dr Moreau" and mixing in elements of "Warlords of Atlantis", if you're like me and a big fan of these cheap and cheerful Italian knock offs you should thoroughly enjoy this.