Alsemero, after glimpsing Beatrice at church, expresses to himself the hope that he can gain her hand in marriage. Outside, in the street, his musings are interrupted by Jasperino. To the latter’s surprise, he learns that Alsemero, whose enthusiasm for travel is common knowledge, is reluctant to undertake a projected voyage to Malta. While they are talking, Beatrice enters, accompanied by Diaphanta, and the four talk in friendly fashion. The mood of Beatrice changes to anger, however, with the arrival of De Flores, her father’s servant, to whom she has a seemingly unconquerable aversion. She makes no effort to hide her feelings from De Flores, who, nevertheless, remains unabashed and continues to follow her about.

Vermandero, Beatrice’s father, passes by and meets Alsemero for the first time. He is pleased to learn that the young man is the son of an old friend of his, a battle companion now dead. To Alsemero he gives an invitation to visit the castle of which Vermandero is governor. The invitation is eagerly accepted, but Alsemero’s pleasure turns to dismay when he learns of Vermandero’s determination to wed Beatrice to Alonzo De Piracquo within the next seven days. As they start for the castle, Beatrice drops one of her gloves. In disdain she throws its mate after it rather than accept the glove from the hands of De Flores, who picks it up and offers it to her.

Meanwhile, in another part of Alicante, Alibius is giving instructions to his servant Lollio. Alibius, a doctor, makes Lollio promise to keep an eye on Isabella, the former’s much younger wife. The doctor’s establishment, which includes facilities for the care of madmen and fools, soon increases with the arrival of a new patient. Antonio, enamored of Isabella, chooses to pose as an idiot so that he can be near her. Lollio interrogates Antonio in an effort to establish his degree of stupidity, but Antonio cleverly parries the servant’s questions.

With the help of Jasperino and Diaphanta, Beatrice and Alsemero communicate with each other and arrange a secret meeting. De Flores, coming to announce the arrival of Beatrice’s suitor, Alonzo, is cruelly railed at, but he equably prolongs the interview to be in Beatrice’s presence. His doggedness arouses in her a vague presentiment of evil, which is quickly dismissed when she rallies herself to face Alonzo. He and her father reluctantly agree to her request for a three-day postponement of the wedding. Her behavior prompts Alonzo’s brother, Tomaso, to utter the warning that Beatrice is not in love, but Alonzo shrugs off any intimation that the marriage is not wise.

Beatrice and Alsemero confess their mutual affection. Beatrice, however, refuses her lover’s offer to engage Alonzo in a duel because she fears that his death or punishment would be the result of such an affair. Instead, she suggests another scheme to get rid of Alonzo, with De Flores serving as a possible tool through whom to work her will. Seeking him out, she gains his consent to help her, but she does not know the price that he expects her to pay. Fate takes a hand in their plotting when Alonzo presently asks De Flores to guide him about the castle’s obscure maze of passageways. De Flores cozens Alonzo into disarming himself, then kills him with a rapier previously hidden behind a door. Before disposing of the body, he cuts off a finger adorned by a diamond ring.

Back at the house of Alibius, Isabella complains to Lollio about the strict watch under which she is kept. Out of curiosity, she prevails upon him to let her visit the quarters reserved for the madmen and fools. There she meets Franciscus, who, like Antonio, is one of the gentlemen from the castle of Alicante with amorous designs upon Isabella. He manages to convey his feelings to her, and she reflects that, after all, a lady need not leave her home if she has any desire to stray from virtue. Lollio, infected by all this romantic intriguing, forgets his master’s commission and makes advances to Isabella, but she repulses him.

De Flores, to prove that he did her bidding, brings the finger of the murdered Alonzo to Beatrice. Refusing her offer of gold, he threatens her with exposure for her part in the crime if she refuses to reward him with her love. Beatrice, twist and turn as she might, can find no avenue of escape from his relentless blackmail, and eventually she yields to his desires.

Vermandero misunderstands the sudden disappearance of Alonzo; angered, he allows his daughter to make a hasty marriage to Alsemero. Nevertheless, he begins to wonder about the prolonged absence from the castle of Antonio and Franciscus; they are sent for, to be questioned about Alonzo. A few hours before her wedding night, Beatrice begins to fear Alsemero’s discovery that she is no longer a maid. Pleading timidity, she persuades Diaphanta, who is still a virgin, to act as her substitute in Alsemero’s bed during the early part of the night. Diaphanta is far from displeased to be asked this favor, even without the gold with which her mistress promises to reward her.

Elaborate nuptial celebrations are planned for Beatrice and Alsemero. As part of the entertainment, Vermandero requests that Alibius rehearse some of his madmen and fools so that they might perform a weird dance for the amusement of the assembly. Alibius decides that he will let Isabella accompany him to the castle for that event. Meanwhile that lady, attracted to Antonio, disguises herself briefly as a madwoman to converse with him. Lollio plays a prank on Antonio and Franciscus by pretending, to each, that Isabella will reward him for getting rid of the other.

Diaphanta, pretending to be Beatrice, amorously overstays her time with Alsemero, so that her impatient mistress becomes first dismayed, then suspicious, and at last vengeful. At the suggestion of De Flores, she agrees to Diaphanta’s death. A fire is set, to create confusion and arouse Diaphanta from the marriage bed. The unfortunate young woman is followed to her own bedroom and slain by De Flores.

Franciscus and Antonio are apprehended and charged with the murder of Alonzo, since it is learned that they entered Alibius’s house in disguise on the day of Alonzo’s disappearance. Beatrice and De Flores finally bring about their own undoing, however, after Alsemero’s discovery of their secret meetings makes him suspicious. Under his questioning, Beatrice breaks down and confesses. Although she pleads her love for him as an excuse for the crime, Alsemero, shocked, takes her and De Flores into custody. The pair are unwilling to face trial; De Flores gives Beatrice a fatal wound and then stabs himself, unrepentant to the last.