In the midst of a war between Venice and Genoa, Marco Polo, a citizen of Venice, is imprisoned by the inquisition and is charged with heresy.The Genoans, whose real interest is seizing control over Venice's lucrative trade routes, use Polo as a pawn and under cover of the inquisition, extract from him a confession that will convince the pope to declare a papal interdiction on Venice.The Venetians call for the French army to assist them in destroying Genoa's plot.