It's exactly what the title tells you...an island inhabited by fishmen. Shipwrecked doctor Claudio Cassinelli and crew land on the island, they're either picked off by the fishmen or roped into working for treasure hunting lunatic Richard Johnson. Cassinelli discovers that Johnson, who believes he's found the lost city of Atlantis, has been keeping disgraced scientist Joseph Cotten and his daughter Barbara Bach hostage for 15 years so the fishmen can uncover a treasure trove beneath the sea. Cotten, of course, is a complete madman. Bach and Cassinelli have great chemistry. This insanity was directed by Sergio Martino and is not, surprisingly, without merit. It's fast paced, reasonably well acted and the fishmen look pretty convincing (though it's unlikely anyone could prove that these things DON'T look like actual fishmen). There's an excellent music score by Luciano Michelini.