A lowly-born girl from Navarre, Anita, is in love with a soldier, Araquil. Araquil's father, Remigio, finds Anita unacceptable and insists she pay a sum of two-thousand duros as a dowry, knowing she will not be able to raise the money. Hearing this, Anita sings a song of lament while the commander Garrido sings of his hate towards the enemy Commander, Zuccaraga (after hearing of his friend's death by Zuccaraga's hands). Anita hears Garrido, and proposes that she kill Zuccaraga for a sum of two-thousand duros. Garrido, though wary and suspicious, accepts the offer. Garrido asks for her name, but she only dashes off saying "I am only the 'girl from Navarre.'" Anita is spotted going to the enemy camp by the soldier, Ramon. When Ramon hears that Araquil is looking for Anita, he (Ramon) tells Araquil that she is in the enemy camp. Ramon misinterprets this, thinking that Anita is actually a spy, and Araquil thinks that she has gone to see a secret lover in the enemy camp.

After an orchestral intermezzo, Anita is successful in killing Zuccaraga and obtains her reward, but is told to swear not tell anyone. Just then, Araquil is brought in, having been mortally wounded searching for Anita. He confronts Anita who, under oath not to tell anyone, can only say "I did nothing wrong." When he sees the money she has received, he accuses her of selling herself, to which she violently objects. Then Remigio, Ramon and the doctor appear on the scene. Bells can be heard in the distance and Araquil asks his father why they are ringing. Remigio tells him that Zuccaraga was assassinated. Araquil looks at Anita again, only then realizing the truth; with the final words "the price of blood! how horrible!" he dies. Anita collapses in horror, and attempts to kill herself. While searching for a suicide weapon, she finds a statue of the Virgin Mary. She becomes crazy with the death of Araquil and speaks as if he were still there. Then, she falls into senseless, wild laughter, as the opera ends with Garrido saying "The madness! The Madness!