Stiffelio is a Protestant Minister whose preaching often takes him away from home. He is married to Lina, who has been seduced in his absence by the young nobleman Count Raffaele. Lina´s father, the retired Colonel Stankar, presumes the affair, but stays silent. When Stiffelio discovers that Lina´s ring is missing, he suspects her of infidelity. Only through her father´s protection the story stays unrevealed. But Stankar challenges Raffaele to a duel, which is stopped by Stiffelio, trying to reconcile them. In his rage Stankar covers up Raffaele´s betrayal. Stiffelio struggles to overcome his jealousy. He finally offers his wife a divorce, ignoring her declaration of continous love for him. When she addresses him as a Man of God, confessing that she was seduced by Raffaele, he reopens his heart for her. Stankar enters the scene with bloody hands: He has killed the seducer. In his following sermon, Stiffelio turns to the story of Christ and the adulteress. Reading aloud Jesus words, he gives a sign of forgiveness to his wife.