Maciste is captured and forced to work as a slave in an African mine.
A scientist who has invented a weapon capable of disintegrating solid matter is kidnapped by a criminal gang, which intends to sell the scientist and his weapon to the highest bidder. A secret agent and the scientist's daughter set out to track down the kidnappers and rescue the scientist.
Rolando, who was close to marrying the daughter of the Doge, is condemned after a highly unfair judicial process and locked up in Venice. Digging a tunnel with the aid of his cellmate, Rolando succeeds to escape.
