 The magistrate has Carlo imprisoned, and he is about to be convicted when Madame Fritz runs in with some documents proving that he is Catherine’s brother.When Carlo becomes aware that he is the tsar's brother-in-law, he introduces Annetta to the imperial couple.Again, not knowing the true identity of the couple, he warns them that the tsar must never see her because she is the daughter of the traitor hetman Ivan Mazepa.