One of the sons of Hercules (Maciste, or, as they call him in this one, Majestis) does battle with a tribe of mole men, who must live underground for fear of being destroyed by the sun and who have vowed to kill all people who live above ground.
Majestis? I don't know how they spell it, but instead of the usual pronunciation for Maciste (My Cheese Steak), we get the pronunciation "My Chest Is". I'm thinking they already used Maciste in one of these "Sons of Hercules" films, and wanted to pretend that this was a different guy with a name that sounded so close to Maciste's that it would be a breeze for the English Language Dubbers Association (who gets a credit here) to handle. Now, if only the English Language Subtitlers Association was on the ball, I'd know the correct spelling. At any rate, it makes sense that these sword-and-sandal heroes would like names that emphasized their chests, given that both Mark Forest and Paul Wynter have prominent ones (as well as Moira Orfei, but that's another story). Yes, the mole men have an evil queen who falls in love with Majestis, as well as an evil courtier with plans of his own. It also has the world's largest manually-operated merry-go-round, an ugly ape creature, bizarre torture devices, a primitive conveyor belt, death by sunlight, and a really big chain. It's all in a day's work for a son of Hercules.