Alicante, Italy, 17th century. Beatrice, daughter of Vermandero, governor of the castle of Alicante, is leaving church when De Flores, servant to Vermandero, asks if he can escort her. She ignores him and walks past. A nobleman, Alsemero, enters and talks to his friend, Jasperino, before going to talk intimately to Beatrice. Vermandero enters with a group of men and talks to Alsemero, whose father he knew.

De Flores, who loves Beatrice despite her disdain for him, makes another approach to Beatrice, who rejects him again. De Flores has a skin complaint and laments that he is not attractive to women. Vermandero enters with another nobleman, Alonzo, suitor to Beatrice. Beatrice, however, prefers Alsemero. Alonzo's brother, Tomazo, warns him about Beatrice, but Alonzo dismisses the warning. Later, however, Tomazo sees Beatrice kissing Alsemero, confirming his suspicions.

De Flores continues to pursue Beatrice and she takes advantage of his interest by engaging him to kill Alonzo for her, intending to rid herself of both Alonzo and De Flores. Vermandero entertains Alonzo at his castle. Alonzo asks De Flores to show him round the castle and in a dark passageway De Flores stabs Alonzo and cuts off his ring finger.

De Flores tells Beatrice he's killed Alonzo and Beatrice tries to pay him, but De Flores insists on sex instead, threatening to tell everyone that she hired him to murder Alonzo if she doesn't comply. Beatrice marries Alsemero but arranges for her maid, Diaphante, to take her place in the marital bed that night so that Alsemero won't discover that she's not a virgin. When Alsemero gives Beatrice a drinking test to prove she's a virgin she fakes the result to convince him she is. Then she gets De Flores to start a fire in the castle in order to prevent Diaphante going to Alsemero. De Flores kills Diaphante, pretending she was killed in the fire.

Jasperino tells Alsemero he saw Beatrice and De Flores together. When Beatrice evades Alsemero's question as to whether she is honest he accuses her of being a whore. She tells him she had Alonzo killed "for his sake". Alsemero tells De Flores he knows Beatrice got him to kill Alonzo. Vermandero brings two men he suspects of having killed Alonzo, but Alsemero says he's found the murderer. Knowing the game is up, De Flores stabs Beatrice while embracing her and then stabs himself.