The film narrates the attempt to overthrow the Republic of Venice in 1618.
A conspiracy hatched by the Spanish Ambassador to Venice and the Viceroy of Naples, the Duke of Osuna, is to take place on the night preceding the Feast of the Ascension, a Venetian religious and secular feast. The ambassador puts the Provencal gentleman Renaud in charge. He is to develop the strategy, while Pierre, who is a renowned pirate and captain of fortune, is to lead the sacking of Venice. Both men also serve the Serenissima Republic. In this enterprise they are to command a group of adventurers, mercenaries, foreign officers and exiles--united by hatred for the independent Republic and by hopes of being rewarded with riches and power by the Spanish King. Pierre is suddenly summoned by the government of Venice to mount guard for a guest of the authorities; therefore he passes the command of the enterprise to Jaffier, his long-time faithful friend and comrade in battles and sackings. Jaffier accepts. But after having met Violetta, the Secretary of the Ten’s daughter, and hearing her praise of, and himself beholding, the beauty of Venice, he begins to question his desire and ability to lead such destruction. Out of compassion for Venice he reveals the plot to the Secretary of the Ten and in return he seeks protection for the lives of his twenty comrades. The Ten give Jaffier this promise, but in the end they betray their promise and execute all of the conspirators including Jaffier, who was to have been exiled.
The Ambassador is removed from office and sent back to Spain.