Paul Massie is really only competent here as physician "Dr. Jekyll" who is becoming increasingly frustrated by the rather linear approach taken by modern science to the issue of the human psyche. Trapped in a loveless marriage with "Kitty" (Dawn Addams) who is much keener on his parasitical friend "Paul" (Christopher Lee), he devotes more of his time to his work and soon develops a serum that turns an unassuming monkey into a maniac! How might it work on a man? Well, we don't have to wait long to find out as his own self-experimentation leads him to discover some rather nasty character traits ...