Isabella, Countess of Suevia, being left a widow, proceeds with indecent haste to take a second husband, Roberto, Count of Cyprus. At a masqued dance given by the bridegroom's friends on the day of the wedding, Isabella falls in love with one of the masquers, whom she discovers to be the Count of Massino [Messina?]. She sends him a letter in which she proffers her love and summons him to her presence. With her paramour she flies to Pavia, where she meets Massino's friend Gniaca, Count of Gaza or Gazia [Gaeta?]. The Insatiate Countess immediately falls in love with Gniaca, who--though at first unwilling to wrong his friend--quickly yields to her blandishments. Returning from a hunting expedition Massino is denied admittance by Isabella. He gives vent to his indignation by penning bitter satirical verses, in which he proclaims to the world her inordinate lust. Enraged at this exposure, Isabella incites Gniaca to slay Massino. An encounter ensues between Gniaca and Massino, but after a few passes the combatants put up their weapons, hold a friendly colloquy, and part in peace. Isabella is furious and resolves to destroy both Gniaca and Massino. She employs the services of a Spanish colonel, Don Sago, who at first sight of her has been violently inflamed with passion. The colonel shoots Massino dead, is arrested, and, being brought before the Duke of Medina, makes full confession. Isabella is condemned to be beheaded. At the place of execution a strange friar requests that he may have private speech with her. The friar is Count Roberto, who has come to pronounce forgiveness, and bid a last farewell, to his erring wife.