The play is set in the Parma region in medieval Italy. Giovanni is an outstanding scholar of impeccable character, and with a promising future. He pours out his heart to a friar he respects, Bonaventura, telling all that has been torturing him for so long. He confesses to being in love with his incomparably beautiful sister, Annabella. Bonaventura rebukes him, but Giovanni's feelings remain as strong as ever and he conveys them to his sister. On hearing what he says, Annabella, too, confesses her love for him.

They consummate their love, but their days of happiness are numbered when it becomes apparent that she is pregnant. To hide her condition, Annabella marries Solanzo, an aristocrat who asked for her hand in marriage. However, Solanzo sees through what she has done, and is able to work out that the child she is pregnant with was fathered by her brother. Solanzo furiously pledges revenge. What does Giovanni do when he realizes the secret is out, and what is the siblings' fate...