Don Jerome arranges to marry his daughter Louisa to Mendoza, an ugly but very rich old fish merchant who has not seen Louisa yet. Jerome hears the young and very poor Antonio serenade Louisa and vows that the wedding of his daughter should take place soon. At the same time Ferdinand, Louisa's brother, laments the capriciousness of his beloved Clara.
Louisa's duenna concocts a plan that will enable Louisa to marry Antonio and at the same time win Mendoza and his money for herself. The idea is that Don Jerome should believe that the duenna is acting as a go-between for the young lovers. Enraged, he will, of course, dismiss her and Luisa, disguised in the duenna's clothes will escape and elope with Antonio and the duenna herself, disguised as Louisa, will marry Mendoza.
After escaping, Louisa meets with Clara who, feigning distress at her lover's (who is Louisa's brother Ferdinand) over-ardent behavior, has decided to take refuge in a monastery. Louisa, this time disguised as Clara, asks Mendoza, who has never met her, to help in finding her lover Antonio.
At Don Jerome's house, Mendoza meets the duenna, disguised as Louisa, and is won over by her flattery and the two decide to elope.
Antonio meets the real Louisa and Mendoza, still ignorant of her true identity, smiles at the young lovers. Don Jerome, interrupted in his amateur music-making, unwittingly blesses the separate marriages of Louisa and the duenna. Clara still pines for Ferdinand.
Mendoza and Antonio arrive at the monastery where the monks sing a boisterous drinking-song. Ferdinand also arrives and, believing that Antonio is about to marry Clara, starts a fight. Confusions as resolved and the monks bless all three marriages. At the wedding ball planned for Louisa, Don Jerome learns of the turns of events and is eventually reconciled, he realizes that his daughter has married a pauper but his son has married a rich heiress.