After a series of scientific experiments directed towards freeing the inner man and controlling human personalities, the kindly, generous Dr Henry Jekyll succeeds in freeing his own alter ego, Edward Hyde, a sadistic, evil creature whose pleasure is murder.

A child conceived by a mute servant girl transforms from an innocent youth to a killer beast at night with uncontrollable urges.

The powers of good are pitted against the forces of evil as the Duc de Richelieu wrestles with the charming but deadly Satanist, Mocata, for the soul of his friend. Mocata has the knowledge and the power to summon the forces of darkness and, as the Duc de Richelieu and his friends remain within the protected pentacle, they are subjected to ever-increasing horror until thundering hooves herald the arrival of the Angel of Death.