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 in himself, but also some facts about the behaviour of those around him. Can he rein in his new alter ego, the debonaire and manipulative ("Mr. Hyde") or will that more dominant, charismatic, character prevail and drive him to dastardly deeds? Christoper Lee is quite good as the rather odious sponger and the story progresses colourfully and without too many silly visual effects - but it's all a bit slow and wordy. It takes too long to get going and though it does deliver quite cleverly in the last twenty minutes or so, the rest of it is a little too procedural. Still, Terence Fisher does enough to keep this intriguing tale of man's inhumanity to himself running along well enough to engage for ninety minutes.
