Maciste arranges for himself and his new friend Bangor to be captured by a mysterious band of white-clad marauders and taken to an underground city. There the two are forced to turn an enormous wheel along with other captives as part of a gold-and-diamonds mining operation. The underground city's queen, Halis Mosab, takes note of the handsome, muscular Maciste and chooses him to be her consort if he can meet such challenges as saving the kidnapped Princess Saliura from a gigantic ape. Maciste kills the ape and carries Saliura back to the aboveground world. The white-clad marauders can't follow them if it means being caught in the sunlight which instantly dissolves them. Maciste then returns to the underground city to save the other captives. Meanwhile, high priest Kahab informs his son, Katar, that their queen is not one of them but, unbeknownst to her, was kidnapped as a small child from the 'world above.' If Katar can marry her, their children can be 'normal' and can live in the world of sunlight. Katar sees Maciste as his enemy and is pleased when the re-captured Maciste is now subjected to a test of strength which will not only kill him but also Bango and another captive named Loth who is in love with the Princess Saliura.