The Tyrol, first half of the 17th century.
Luisa, the daughter of an old soldier, is in love with a young man she knows as Carlo but who is actually Rodolfo, son of the local lord, Count Walter. The two lovers proclaim undying fidelity, but Miller, Luisa's father, is dubious, and his fears are confirmed when Walter's retainer, Wurm, who also loves Luisa and hopes to marry her, reveals Rodolfo's true identity. In Walter's castle, Wurm tells his master of Rodolfo's love for Luisa. Walter resolves to end their relationship, because he hopes to have his son marry his niece, a widowed duchess, Federica. Left alone with Federica, Rodolfo reveals that he loves another, but the duchess, who has worshiped him since childhood, refuses to break their engagement.
At home, Miller tells Luisa that Rodolfo has deceived her and is about to commit to a wealthy marriage. The young man, however, comes to plead his sincere love. When Walter storms in shortly afterward and is about to have both Luisa and her father consigned to prison, Rodolfo secures their freedom by threatening to reveal how his father, with Wurm's assistance, murdered his cousin to gain his present position.
Luisa learns that her father has been jailed for insulting Walter. Wurm tells her the only way she can save Miller is to write a letter admitting she sought Rodolfo for his wealth, and pledging herself to Wurm. After doing his bidding, she learns she must go to the castle and declare her love for him before the duchess. Wurm presents Luisa's letter to Walter, and the two plot to send it to Rodolfo. Wurm then brings in Luisa. Goaded on with threats by Wurm and Walter against her father, she professes her love for Wurm to Federica. Rodolfo receives Luisa's letter in the castle courtyard. In despair he is about to attack Wurm when Walter appears and persuades him that marrying Federica will be the best way for him to avenge Luisa's treachery.
Miller, released from prison, tries to comfort Luisa. The two agree to leave the village the next day. As Luisa prays, Rodolfo enters and pours a vial of poison into a decanter on the table. He confronts Luisa with the letter. When she cannot deny she wrote it, Rodolfo asks her to pour him a drink; when he says it tastes bitter, she swallows some too. Rodolfo tells Luisa the cup was poisoned, and she, released from her vow, tells him the truth. As Luisa expires in Miller's arms, Rodolfo, with a final effort, kills Wurm.