Adelaide takes possession of an island in the Pacific inherited from her deceased husband. She is opposed by the chieftain and a convict. To get the island's citizenship she tries to marry the convict, but he prefers to turn himself in.

Syracuse lies between the warring nations of Rome and Carthage; as long as the balance of power between the nations remains intact, both nations are willing to preserve the neutrality of Syracuse. However, Rome has now gotten the upper hand in its struggle for power. The fate of Syracuse lies in the hands of its leader, the famed inventor and scientist Archimedes.

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...