In 17th-century Tryol, Luisa is the ingenuous daughter of the old solider Miller, and Rodolfo the son of the local seigneur.
They love each other passionately.
When the count Walter hears of this idyllic affair that his son is conducting with the young country girl (whereas he intends his son for his cousin, the duchess Federica), he has Luisa and her father locked up.
In the meantime, the valet Wurm, who is not indifferent to the charms of Luisa, suggests that she write a letter denying her love and follow him.
She accepts the suggestion, which secures her father's release.
Rodolfo learns of the letter and, finding Luisa, forces her to share a cup of poison with him...