Returning home from his latest adventure, Hercules (Reg Park) and his sidekick, Theseus (George Ardisson), are shocked to find their home city has fallen victim to a plague that puts its victims in a trance-like state.  The woman that Hercules loves, Deianira (Leonara Ruffo), is one of the victims and, since she was also the city’s queen, the sinister Lico (Christopher Lee) is ruling in her place.

Hercules consults with the oracle, Medea (Gaia Germani).  Medea says that the plague can only be lifted by the Stone of Forgetfulness, which can only be found in the land of the dead, Hades.  Hercules and Theseus set out for Hades but before they can enter the realm of the dead, they have to perform a quest to defeat a rock monster and retrieve a magic apple from a giant tree.  Nothing is simple in ancient Greece.

The best of all the Hercules films, Hercules in the Haunted World may not have had Steve Reeves in the lead role but it did have Mario Bava behind the camera.  Bava shows what a clever director can achieve just through creative lighting, colorful mists, and detailed set design.  The film has all of the mythological monsters and toga-clad action that you expect from a Hercules film but it also has atmosphere, bleeding plants made from the souls of the dead, zombies, and Christopher Lee.  Lee may not be playing a vampire here but he still finds an excuse to drink blood in an attempt to achieve immortality.

Reg Park was a Brit who was inspired to become a bodybuilder after watching Steve Reeves in a competition.  When Reeves left the role of Hercules, Park was cast in his place.  Park only made a total of five peplum films and he was even worse at expressing emotion than Steve Reeves.  Park did have the physique necessary to play Hercules and that was really all that was needed.  Though Park tired of acting, he would still go on to mentor another bodybuilder who was inspired by Steve Reeves and would play Hercules in a film, Arnold Schwarzenegger.