I.i begins outside of the Duchess's house. Lactantio and Aurelia, sworn lovers, are hindered in their match by Lactantio's uncle, the Cardinal. The Cardinal is a religious man who loves chastity better than any other virtue. The Duchess is his ideal, and he has written many sonnets in her honor. Lactantio, the Cardinal's sole heir, fears that a love match between himself and any woman would lose his inheritance from the Cardinal. Lactantio has therefore dissembled that he is a devoutly religious young man who fears women and abjures them. This feigned piety has fooled and pleased the Cardinal. Lactantio conspires with his love, Aurelia, to sneak her into his chambers by some trick.

I.ii The Cardinal is persuaded by 1 Lord to test the Duchess's virtue. As in The Second Maid's Tragedy, the male character mistrusts untried chastity. The Cardinal agrees to advise the Duchess to go forth in the world-if only to prove that her chastity and vow are true and unshakable. 1 Lord, however, has designs upon the Duchess, and his advice to the Cardinal is intended to bring the Duchess out where he might try her for himself.

We see the Cardinal and Lactantio, who continues dissembling misogynistic piety for his uncle's benefit. The Cardinal is well pleased with his "work" in Lactantio. There is a woman, however, who is really Lactantio's mistress, waiting upon Lactantio in the guise of his page. She informs Lactantio that she is pregnant with his child. She asks him to make good on his promise to marry her, but he responds that he has made many such promises to many such women and that she must take what he chooses to give her. The page brings Lactantio news that a gentleman has come from Rome to see him. The gentleman is Aurelia in disguise (this being Lactantio's plan to be close to his lover without the Cardinal's suspicions being aroused.) The Cardinal is fooled by the disguise and bids the "gentleman" welcome before leaving to see the Duchess.

Aurelia tells Lactantio that she escaped from her father just in time. Her father had planned to marry her off to the old Governor of Milan. Her father and the Governor enter at that moment to speak with the Cardinal. They meet Lactantio and the "gentleman" and speak with them. The father sees through his daughter's disguise at once, grows angry, and he and the Governor remove her to the fort so that she shall not escape him again. He upbraids Lactantio for his hypocrisy before leaving. Lactantio determines to retrieve Aurelia.

I.iii takes place at the Duchess's house. The Duchess and her servant, Celia, enter on a balcony. The Duchess expresses herself satisfied with her vow. The Cardinal enters and goes about his mission of persuading the Duchess to walk forth and test her vow in the world. He argues that only then will she "astonish sin" and prove a worthy example of chastity in the world-a chastity that can withstand temptation. The Duchess agrees, and the Cardinal leaves to participate in the triumph of Andrugio, the returning war hero. The Duchess gets one look from her balcony at the returning Andrugio and immediately falls in love with him. Andrugio, however, is looking for the woman of his desires: Aurelia.

I.iv introduces us to the "fool" of the play, Donaldo. The page asks him to shake out Lactantio's shirt, but Donaldo refuses until the "young boy" sings him a song. The page says that "he" cannot sing, but finally relents. By the end of the scene we learn that Donaldo is unsatisfied with his part in Lactantio's service and has determined to run away.

II.i finds the Duchess plotting to win Andrugio. She tells Celia that she has fallen in love with Lactantio. When the Duchess tells the Cardinal he is astonished. He first determines to disinherit Lactantio. After the Cardinal storms out, thus upsetting her plans. She had hoped that the Cardinal would accept her choice to break her vow as it was broken with his nephew, the only man he would not harm. The Cardinal's violent reaction leads her to believe that her trick has failed.

II.ii The Cardinal reconsiders his rashness and realizes that Lactantio had no part in causing the Duchess's to recant her vow. He decides not to punish Lactantio. He instead decides, as the Duchess had hoped, to give his blessing to the union.

III.iii takes place in the castle where Aurelia is being held. Andrugio comes in disguised in order to see her. Aurelia does not recognize him and first thinks about sending a love letter by him to Lactantio, but she repents when she remembers that Andrugio is in the city (and this man might be one of his spies). Andrugio reveals himself to her, and she feigns great joy in seeing him. She conspires with him to gain her freedom. He has smuggled in some gypsy clothes for her to wear in order to slip past the guards. She is then to infiltrate the gypsies in Milan in order to escape apprehension when her disappearance is discovered. Andrugio will come for her at the gypsy camp. Aurelia approves the plan, but has a secret counterplan to escape Andrugio as well and fly to Lactantio after she gets away from her father and the Governor.

III.i The page begs Lactantio to marry her to avoid both of their shame, but he refuses. The page cries, as she is wont to do throughout the play. Her crying, in fact, leads the Cardinal to suspect that Lactantio mistreats the boy.

Donaldo determines to run away and join the gypsies because they are the wittiest thieves and will appreciate his special talents in foolery and thievery.

The Cardinal, entering to find the page weeping, tells "him" not to fret, that he has arranged to have the page given to the Duchess as a gift. This, the Cardinal hopes, will remove the sweet-faced boy from the poor treatment of Lactantio. The Cardinal tells Lactantio that the Duchess has favored him and that he has approved the match. Lactantio, only half acting, tells the Cardinal that he has no interest in women and that he does not wish to be favored by the Duchess. Left alone, though, Lactantio realizes all the benefits of marrying the Duchess and resolves to be her man.

Meantime, the Cardinal, wishing to smooth the way for the match between the Duchess and his nephew, meets with the Lords. He tells them that the Duchess's vow was rashly taken, was made under the duress of her husband's impending death, and has been kept as long as such vows should be kept. He convinces the Lords that she should no longer be held to her vow. 1 Lord, who has ulterior motives in his designs on the Duchess, eagerly accepts this advice. The Cardinal pretends that the Duchess is still resolute to her vow and counsels the Lords to take every opportunity to dissuade her from it. This ploy is mainly to make the Duchess seem a fitting match for his nephew, and not a wanton, but also to throw off suspicion that he has encouraged a match between the Duchess and Lactantio.

III.ii The Duchess learns from Celia that Lactantio and Andrugio are secret enemies. When Lactantio comes to call, the Duchess plays hard-to-get for a moment but yields quickly to her supposed love. She gets Lactantio to confess that the only enemy he has in the world is the general, Andrugio. The Duchess tells Lactantio that they will undo the general. She makes Lactantio write a love letter to her in Andrugio's handwriting-this she says she will use to discredit Andrugio. Lactantio agrees. This is actually the Duchess's plan to catch Andrugio for herself.

IV.i begins in the gypsy camp. Andrugio is waiting for Aurelia. Lactantio enters with guards, sent by the Duchess, and arrests Andrugio in the Duchess's name. Aurelia enters disguised as a gypsy. She meets the fleeing Donaldo, who scorns his service to Lactantio. They both fall in with the gypsies, who accept them and make Aurelia Donaldo's "doxy". They tell Donaldo that, because of his skill and background, he will shortly become captain of the gypsies-they read as much in his palm. The Governor and Aurelia's father enter looking for Aurelia. They do not penetrate her disguise this time, and she tells their fortunes: they will never find what they seek.

IV.ii returns to the Duchess's home. The Cardinal and Lords make a show of talking her out of her vow-especially 1 Lord. The Cardinal makes a gift to her of the page. The Duchess likes the pretty "boy" at once and determines to raise "him" properly. She sends for singing and dancing masters to educate "him." Lactantio brings in Andrugio under arrest. Andrugio is angry at having been taken, and especially angry at having been taken by the foppish Lactantio. The Duchess wishes to speak to Andrugio alone. She shows Andrugio the forged love letter and asks him to explain. When he denies it, she refuses to believe him and tells him that she yields to him, that he may have her. Andrugio despairs that this love of the Duchess's will overthrow his love for Aurelia-he fears he is trapped in a match that he cannot escape.

V.i finds the page at "his" singing lesson. She is close to delivering her baby and the singing is torture on her. She makes it through, however, only to be taken over by the dancing master. In the midst of the lesson she falls down in labor and calls for a midwife.

V.ii finds Celia telling the Duchess that she has seen Andrugio kissing a vile gypsy. The Duchess is enraged that her lover should prefer an ugly brown woman to her. She calls for the gypsy to be brought in. Aurelia, still disguised, is ushered in. The Duchess believes that Andrugio is insane to love such a tawny creature. She dismisses the "gypsy" and calls for Andrugio. She confronts him with her discovery. Andrugio denies that he loves so unworthy a creature as she describes. The Duchess calls for the woman to be brought in again. This time Aurelia enters in her own shape. The Duchess sees that she has not been preferred to a lesser but to a better woman; her vanity appeased, she grants Andrugio his freedom to marry Aurelia.

Just then Lactantio enters. Aurelia scorns Andrugio and runs to Lactantio. Lactantio scorns Aurelia and says he prefers the Duchess. Aurelia begs Andrugio to take her back to him. Andrugio accepts her apology and takes her back.

The Duchess scorns Lactantio and calls out the page in female dress. She reminds him of his duty to her. He must marry the woman. The Cardinal is astonished to learn how Lactantio has acted behind his back and disowns him. The Duchess calls for mercy and bestows ten thousand ducats from her treasury as the page's dowry. The Cardinal relents and reinstates Lactantio to his good graces for mercy's sake. The Duchess swears to keep her vow of chastity, to give up her worldly life, donate all her money to the church, and join a convent in order to live with her vow forever.